If you're a 40-year-old investor, you're probably not planning on retiring for another 20 years or more. While that may be bit of a downer for those looking forward to the freedom that retirement can bring, the positive side is that it gives you more than two decades to grow your investments as you prepare for that fateful day.

Yet if you're in your 40s, your tolerance for risk may not be what it was back in your 20s. You may want growth, but don't want to chance losing your shirt to get it.

Fortunately, there are some businesses that, thanks to their strong competitive advantages, offer an excellent combination of growth potential and relative safety. Read on to learn about one of the best.

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Zillow Group (NASDAQ:Z) (NASDAQ:ZG) is the largest online real estate marketplace. The company's collection of brands includes its namesake Zillow.com site, as well as popular real estate properties StreetEasy, HotPads, and Naked Apartments. Together, these sites allow house and apartment hunters to search more than 100 million listings, compare Zillow's proprietary Zestimate home values, and connect with local real estate professionals.

Zillow solidified its position as the No. 1 player in online real estate by acquiring No. 2 rival Trulia in February 2015. The resulting company's reach is unmatched, commanding about two-thirds of the online and mobile real estate audience, and nearly three quarters of the mobile-only category , according to comScore data. More than 140 million unique users visited Zillow's mobile apps and websites in the fourth quarter -- an increase of 13% year-over-year. Perhaps most telling of all, more Americans now search "Zillow" than "real estate" according to Google Trends. 

Advertisers follow audience, and real estate professionals are willing to pay hefty sums to reach Zillow's swelling user base. Yet even after its impressive growth, Zillow Group has only scratched the surface of its total market opportunity. Zillow earned $604 million in premier agent revenue in 2016 and delivered nearly 17 million leads to its customers. It estimates that those leads helped agents earn roughly $4.4 billion in commissions, which represents only about 5% of the more than $87 billion in total real estate agent commissions that were paid in the U.S in 2016. "The opportunity in front of us remains massive," said CEO Spencer Rascoff when speaking about this topic during Zillow's fourth-quarter earnings call.

The trends are certainly in Zillow's favor. In 2015, Zillow delivered 12 million leads to agents, which drove about $3.2 billion in commissions, or about 3.9% of total U.S real estate commissions that year. So Zillow's share of this massive market is already growing rapidly. And with more would-be home buyers turning to the Internet to find their ideal property every year -- the National Association of Realtors places the figure at about 44%, up from 36% in 2010 -- Zillow should continue to gobble up market share as agents redirect more of their advertising spending to Zillow's leading online real estate marketplace.

This highlights what is perhaps Zillow's greatest competitive advantage: its powerful network effect. Each additional user increases the value of Zillow's platform, as more potential customers attract more agents, and more agents list more homes for buyers and renters. This virtuous cycle should further expand Zillow's already unmatched reach and scale, and position the company to grow exponentially as ad spending shifts to the Web and mobile devices in the coming decade.

All told, Zillow Group's massive long-term growth potential and dominant competitive position make it an ideal stock for investors in their 40s to consider.

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.