While younger workers are increasingly enrolling in 401(k) plans, many might not be saving enough.

Nearly half (44%) of workers ages 21 to 34 have saved more than $50,001 in their 401(k) accounts, according to a recent survey by personal finance website GOBankingRates, which polled 1,000 working Americans ages 21 and older who have been in their current roles for at least one year.

While $50,001 is a significant amount — exceeding the median personal income of $42,220 per year — it may still fall short of a common retirement readiness benchmark.

Financial firm Fidelity recommends saving at least the equivalent of your annual salary by age 30 and twice your salary by 35. That progresses to three times your salary by age 40, six times by 50, eight times by 60, and 10 times by 67.

For a worker earning the median income, this means having:

• $42,220 saved by age 30
• $84,440 saved by age 35
• $126,660 saved by age 40

Here's a breakdown of how much workers 21 to 35 have in their 401(k)s, according to GOBankingRates' survey:

• Less than $25,000: 19.6%
• $25,001 — $50,000: 31.7%
• $50,001 — $100,000: 32.9%
• $100,001 — $500,000: 10.8%
• $500,001 — $1 million: 0%
• More than $1 million: 0%
• I don't have a 401(k): 5.1%

While these numbers show that many younger Americans may not yet be fully prepared for retirement, it's important to remember they only represent 401(k) balances. Other savings, like individual retirement accounts, brokerage accounts, cash reserves or potential inheritances, could help boost retirement wealth down the road.

It's also worth noting that workers often increase their retirement contributions later in life to make up for lower earnings earlier in their careers, so playing catch-up is common.

As workers get older, their 401(k) balances tend to grow. This may be reflected in the share of workers with more than $100,000 in their accounts, which doubles from about 11% under 35 to nearly 21% by their mid-40s, GOBankingRates' study finds.

However, while catching up in your 30s and 40s is possible, delaying contributions can require significantly higher monthly savings later in life to reach the same retirement balance.

This is because starting later means missing out on the power of compound interest, which allows investments to grow exponentially over time.

If you invest $200 per month starting at age 25, it will grow to around $525,000 by age 65, assuming a 7% annual return. Waiting until age 35 to start would require contributing around $430 per month to achieve the same result.

This is why financial planners strongly recommend starting as early as possible, even with small contributions like $50 per month.

Want to make extra money outside of your day job? Sign up for CNBC's online course How to Earn Passive Income Online to learn about common passive income streams, tips to get started and real-life success stories.

Plus, sign up for CNBC Make It's newsletter to get tips and tricks for success at work, with money and in life.