Millennials are investing like never before, thanks partly to the popularity of the Robinhood trading platform. That will give them decades to watch their investment portfolios grow.

Of course, how much growth accumulates over the years depends on the caliber of the stocks bought and held. The ideal stocks have solid competitive advantages combined with a huge potential market. Here are three growth stocks that millennials should buy right now that check off both of those boxes. 

Young couple at a table looking at a laptop screen.

Image source: Getty Images.

1. PayPal Holdings

PayPal Holdings (NASDAQ:PYPL) has been a major player in the digital payments market for a long time. Now a $200-billion-plus giant, the company still has plenty of room to run.

PayPal's third-quarter results, which were announced earlier this month, proved as much. PayPal reported its strongest quarterly revenue and total payment volume growth ever in Q3. Over the last two quarters, the company has added nearly as many new accounts as it did in all of 2019.

Millennials generally like to invest in shares of companies that market products they use, so they will probably like PayPal. The company's Venmo peer-to-peer payment app is wildly popular and continues to grow in usage as well as revenue generated.

The e-commerce market continues to explode, with the COVID-19 pandemic only accelerating its rise. This obviously presents a huge opportunity for PayPal. However, the company is also positioning itself for growth in the physical setting by introducing QR functionality for its PayPal and Venmo apps. With two tremendous paths for growth, PayPal should make investors who buy now and hold the stock a lot of money over the next decade and beyond.

2. The Trade Desk

The Trade Desk (NASDAQ:TTD) might not seem to fit into the "buy what you know" category as much as PayPal does. However, millions of millennials likely see the fruit of the company's efforts every day.

Any time you see an ad while watching a streaming TV channel, there's a good chance that the ad was bought using The Trade Desk's leading programmatic advertising platform. The company enables advertising agencies to buy digital ads electronically without having to deal with media companies. 

This market, which insiders refer to as the connected TV (CTV) market, is only in its early stages. The Trade Desk is positioned to continue growing at a very fast pace as the CTV market expands in the coming years.

What about competition? Other companies are going to have a hard time keeping up with The Trade Desk. Even though it's the market leader, The Trade Desk plans to roll out a massive upgrade to its platform next year that should be even more attractive to customers. Buying shares of a company with a huge market that refuses to rest on its laurels usually generates great returns.

3. Innovative Industrial Properties

Not every growth stock is a tech stock. Innovative Industrial Properties (NYSE:IIPR) is a great case in point. It's a real estate investment trust (REIT) -- but with a twist.

This IIP focuses on the U.S. medical cannabis industry. It buys properties from medical cannabis operators, then leases them back to the operators. IIP gets a steady revenue stream while its tenants access the capital they need. 

IIP has delivered tremendous growth over the last three years, with its revenue soaring around 1,400% and its earnings skyrocketing by over 3,000%. However, the company still only owns 63 medical cannabis properties in 16 states. With two more states voting to legalize medical cannabis in the elections earlier this month, IIP now has 19 states it could expand to. 

The stock also offers another nice plus to add to investors' returns -- a juicy dividend. As a REIT, IIP must return at least 90% of its taxable income to shareholders as dividends. IIP's dividend continues to rise as its profits grow and currently yields a little over 3%. Millennials might not think too much about dividends, but over time they can increase total returns.