At the end of 2013, new hires had 41 percent of their assets in balanced funds, most of which were in TDFs; 32 percent of all new hires' assets were in funds that automatically reallocate with age.

When accounting for all of the 26.4 million plan participants in the EBRI/ICI database, target-date funds represented 15 percent of plan assets, and 41 percent of all participants had some of their assets allocated in TDFs.

And that's reduced the chances of participants being dangerously over or under weighted in stocks, according to EBRI Research Director Jack VanDerhei.

"The growing use of these funds in recent years, especially among new 401(k) participants, has been accompanied by a marked decrease in young participants with zero equity exposure, and has also been associated with a decrease in older participants with high concentrations in equities as well as a continued reduction in the allocation to company stock among participants."

More participants held stocks in 2013 than at year-end 2007, when stock markets peeked prior to the financial crisis. Younger investors have particularly evolved their allocation habits, as two-thirds of participants in their 20s had more than 80 percent of their accounts invested in stocks. Back in 2007, only half of younger participants were as aggressively allocated.

All told, 90 percent of investors had some portion of their accounts in stocks at year-end 2013.

The report breaks down the relationship between account balances and participants' age, tenure and salary.

Half of the account balances with less than $10,000 were held by participants in their 20s and 30s, and 60 percent of accounts with more than $100,000 were held by those in their 50s and 60s.

And 74 percent of those with more than $100,000 in their account had more than 10 year of tenure with their sponsor.

As might be expected, tenure in a plan affects account balances.

But the EBRI/ICI data proves just how dramatically.

The average balance of a participant in their 60s with two years in the plan was $31,582. For those with 30 years under their belt in the same age group, the average balance was $248,397.

That discrepancy held true for mid-career workers as well. Participants in their 40s with two years of service had an average balance of $19,104, compared to $154,228 for those with 20 years of tenure.

For all 401(k) investors, equity funds have become increasingly more favorable since the financial crisis. In 2013, 44 percent of account balances were in equity funds, compared to 39 percent in 2012, 37 percent at the end of 2008.

At year-end 2007, before the financial crisis ravaged markets, 48 percent of total balances were in equity funds.

When accounting for all sources of stock investment—equity funds, equity portion of balanced funds and company stock—66 percent of all 401(k) assets were invested in stocks at year-end 2013.