NEW YORK (MainStreet) -- Of nearly $25 trillion in total U.S. retirement accounts, more than $7 trillion is held in IRAs. A 40-year-old savings innovation, the venerable accounts are now nearly half comprised of mutual funds (48%), with a great deal of the assets coming from 401(k) rollovers.

So, how are these multi-layered and often restrictive retirement savings accounts doing? The Employee Benefit Research Institute has compiled four consecutive years of data on 25.8 million accounts with total assets of almost $2.5 trillion. The study shows a widely varying degree of savings success.

From 2010 to 2013 the average balance for those with IRA accounts during each of the years increased more than 32%. But nearly one-quarter of investors had increases of less than 3% over the same period. However, the highest quartile of balance changes exceeded 57%.

Roth IRAs saw even larger gaps in balance improvement. The lowest quartile of balance changes were over 26%, but the highest quartile account holders saw balances improve by more than 84%.

Investment performance – and probably more importantly, contributions – make the difference. Over that same four-year period, only about 11% of traditional IRA owners made annual contributions. But Roth IRA owners were better savers: 37% made contributions over the four-year period.

The Roth IRA is where the young money is going. Nearly one-quarter (23.9%) of Roth accounts receiving contributions were owned by individuals ages 25 to 34. A skinny 7.5% of traditional IRAs saw contributions from the same group of young adults.

Average annual contributions for all IRAs increased from $3,335 in 2010 to $4,145 in 2013.

“Just over 45% of those owning IRAs had less than $25,000 in their accounts at year-end 2013,” write Craig Copeland, senior research associate at EBRI and lead author of the report. “For all IRAs combined, 27.2% of individual owners had balances of $100,000 or more.”

IRAs, along with just about every other investment account on the planet, suffered in 2011. But they rebounded well from the setback. Combining all accounts for the same person, the average IRA balance dropped from $91,864 in 2010 to $87,668 in 2011. But by the end of 2013, the average balance was up to $119,804 -- an increase of 30.4% from 2010 to 2013, and 14.1% from 2012 to 2013.

“The increases found in the average balances in 2013 are likely to continue, as is the importance of IRA assets for individuals during retirement,” Copeland said in a statement. “With growing 401(k) plan balances and IRAs being a popular destination for 401(k) assets when people change jobs or retire, the amount of income derived from IRAs will grow significantly as a supplement to Social Security for the Baby Boom and Gen X generations.”