Are you in your 20s and anxious about investing? Don't be! In fact, if you're still at a relatively young age, you're better positioned than anyone else to invest for a happy and prosperous retirement. At that age, shying away from stocks and investing can actually be counterproductive to your long-term financial stability.

Even if you don't have very much investable money today, saving and investing even a little bit early on can provide large dividends -- literally and figuratively -- down the road. That's because time is the most important concept in investing, thanks to the miracle of compound interest, which Albert Einstein described as the eighth wonder of the world. And if you're young, not only do you have the luxury of time, but you also have the luxury of being very aggressive in your investing style early on. That's because if, say, a high-risk, high-reward investment doesn't work out, you still have time to make up for it with a lifetime of wages to come.

But are you still intimidated by picking individual stocks? Don't worry -- there are myriads of different exchange-traded funds (ETFs) out there that can give you broad exposure to certain sectors, styles, indexes, and/or risk-reward profiles to meet your investing goals, and usually with very low fees.

Thus, if in your 20s, that means you should lean toward ETFs that contain aggressive growth stocks and/or smaller stocks with the potential for massive upside over many years. Here are five such ETFs, each of which has the potential for big long-term gains.

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Invesco QQQ Trust (QQQ)

Let's start with one of the more basic building blocks for any growth-oriented investor's portfolio, the Invesco QQQ Trust (NASDAQ:QQQ). The QQQ is kind of like investing in the broader market of large-cap growth stocks, as it aims to mimic the performance of the technology-heavy Nasdaq 100. The Nasdaq is home to most of the world's largest technology stocks such as the famed FAANG internet stocks, along with many large semiconductor stocks as well, and that's not such a bad place to be, given the long-term trends in cloud computing, artificial intelligence, e-commerce, and the Internet of Things. Many stocks in the QQQ are key players in all of these big tech trends, giving it a very positive long-term outlook.

The QQQ is actually up nearly 13% this year, compared with a 0.2% decline for the broader S&P 500. The difference can probably be attributed to the QQQ's huge 43.85% weighting in technology, compared with the S&P's lower 22.6% technology weighting. Technology has been a strong outperformer amid the coronavirus this year, as many investors think the pandemic could lead to an acceleration of the aforementioned tech trends, as companies adapt to remote work, e-commerce, telemedicine, and other tech-based solutions.

Large-cap technology has been the best place to be over the past decade or so, and it looks like a solid bet to continue its growth over the next decade as well. In fact, the biggest risk for the QQQ may not be the stocks within the index themselves, but the potential for being so successful that the government steps in with antitrust measures.

Vanguard Information Technology Index Fund ETF Shares (VGT)

While the QQQ is quite tech-heavy compared with the overall market, let's say you wanted to go all in on technology. Is there a low-cost ETF for that? You bet there is. The Vanguard Information Technology Index Fund ETF Shares (NYSEMKT:VGT) offers a broad exposure to large, mid-cap, and smaller companies almost entirely in the technology sector, with an 87.56% allocation to technology stocks. And while some of the stocks within this ETF may "officially" be classified as a financial or an industrial stock, there's probably a strong overlap with technology and these industries in some way.

So if you'd really like to go all in on tech and eschew tangential categories such as consumer discretionary stocks or communications stocks you might find in the QQQ, the VGT could be for you. Even better? It comes at a rock-bottom expense ratio of 0.10%.

One thing to note: This ETF is very concentrated in Apple (NASDAQ:AAPL) and Microsoft (NASDAQ:MSFT), which together make up about 37% of the total ETF -- not that that is necessarily a bad thing, given each company's long-term outperformance, but it does bring some single-stock risk should anything go wrong with these two companies.

Vanguard Small-Cap Growth Index Fund (VBK)

Small-cap stocks have underperformed large caps this year and over the past decade or so, but that is actually an exception to the rule. Between 1926 and 2006, small-cap stocks outperformed large-cap stocks by an average of 2.3 percentage points annually. That could make a huge difference when compounded over 20, 30, or 50 years. When you think about it, that makes sense; after all, a smaller company has more room to grow versus a large stock, whose growth can be impeded by the law of large numbers.

Of course, that has not been the case over the past decade or so, as the largest companies in the market have seemingly gotten stronger and stronger. However, trends can reverse over time, and since small caps have historically outperformed in previous decades, perhaps they will again.

For those looking for an underperforming ETF that could bounce back in an even bigger way, look no further than the Vanguard Small-Cap Growth Index Fund ETF Shares (NYSEMKT:VBK). I'd recommend the growth-oriented small-cap ETF because growth stocks seem more compelling than a broad cross section of small caps or value small caps these days.

For instance, the VBK has a 29.76% allocation to technology and a 24.75% exposure to healthcare stocks. These are both overweight allocations compared with the straight-up Vanguard Small-Cap Index Fund ETF Shares (NYSEMKT:VB), which has more exposure to, in my opinion, less attractive long-term sectors such as financials and energy.

ARK Innovation ETF (ARKK)

Want something even more aggressive than the tech-based growth ETFs mentioned above? ARK Innovation ETF (NYSEMKT:ARKK) is an actively managed ETF, which means there are managers making stock selection and allocation decisions, as opposed to an ETF that aims to merely mirror an index. Nevertheless, actively managed ETFs still offer benefits such as tax efficiency, liquidity, and transparency that are typical of an ETF. That may sound like the best of all worlds, but an actively managed ETF usually has a higher expense ratio. That's the case with ARK, which charges a 0.75% expense ratio, much higher than, say, the 0.20% expense ratio for the QQQ.

ARK is managed by Catherine Wood and her team and invests only in disruptive, next-generation technology companies that may or may not have much in the way of current earnings, but with the potential to disrupt or create entire new markets. These include things such as next-generation internet companies, genomics, robotics, cryptocurrencies and fintech, 3D printing, mobility-as-a-service, and other futuristic gadgets and services.

Recent enthusiasm for disruptive companies has led ARK to absolutely crush not just the broader market but also the aforementioned technology indexes above as well. Year to date, ARK is up a whopping 27.5%. That is partly attributed to the performance of Tesla (NASDAQ:TSLA), which is the ETF's largest position at 11.7% of the portfolio. However, ARK also includes other Fool favorites such as fintech firm Square and genomics company Illumina among its top holdings.

Of course, with that huge upside comes the potential for volatility and losses just as big over a similar period of time. But hey! You're in your 20s! You can handle volatility. And since you're a Fool, you have the long-term perspective needed to weather these ups and downs and hold for big potential long-term gains.

Schwab Emerging Markets Equity ETF (SCHE)

As a young person, you should also embrace your inner explorer. That means allocating a portion of your portfolio to emerging overseas markets. After all, since you're interested in fast-growing companies, why allocate investments toward the fastest-growing economies and countries?

For emerging markets exposure, I'd recommend the Schwab Emerging Markets Equity ETF (NYSEMKT:SCHE). The SCHE gives investors broad exposure to some of the best companies in China, India, Taiwan, and other growth markets, aiming to mirror the total return of the FTSE Emerging Markets Index at a low expense ratio of just 0.11%. Its largest holdings include Chinese giants Alibaba (NYSE:BABA) and Tencent (OTC:TCEHY), along with leading Taiwanese semiconductor foundry Taiwan Semiconductor Manufacturing (NYSE:TSM).

While the index is down about 10% on the year, as U.S. stocks have strongly outperformed emerging markets stocks of late, that's not always the case. Furthermore, these countries are growing faster than the U.S. overall.  In 2019, China's GDP grew 6.1%, India's GDP grew 4.2%, Taiwan's GDP grew 2.7% -- all higher than U.S. GDP growth of 2.3%. That means that while the companies in this ETF have perhaps more risk due to being in emerging markets, they also have greater growth potential as well.

One current significant risk to ponder is U.S.-China trade tensions, which may have contributed to some of this ETF's recent underperformance. However, over the long-term, China seems poised to grow by leaps and bounds, with its 2 billion people, and many of them just now entering the middle class. Assuming you're willing to take on this risk for a portion of your portfolio, the SCHE could make an excellent choice to obtain top-quality emerging market exposure in any twentysomething's portfolio.

Don't be timid in your 20s

Your 20s are a time to be aggressive with your investments, and each of the tech-based ETFs above could be a fine way to jump-start your journey to a comfortable retirement -- perhaps even an early one if you choose right!

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.