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Americans with investment accounts grew a lot richer last year thanks to the booming stock market—but the 65-plus crowd enjoyed the biggest increase in savings for retirement of any age group.

Total U.S. household investable assets (liquid net worth, not including housing wealth) surged 16% to $41.2 trillion in 2013, according to a report published Wednesday by financial research firm Hearts & Wallets. That far exceeded annual gains that ranged from 5% to 12% in the post-Recession years of 2009 to 2012.

But when it came to retirement savings, older investors saw the biggest gains in IRA and 401(k) assets: Retirement assets for people age 65-74 rose from $2.3 trillion to $3.5 trillion in 2014, a new high.

What's fueling the growth? Well, a lot of people 65 and older aren’t retiring. So they’re still socking away money for their nonworking years. Meanwhile, others who have quit work are finding they don’t need as much as they thought, so they continue to save, according to Lynn Walters from Hearts & Wallets.

As attitudes about working later in life change, so does the terminology of what people are saving for, Walters says. Rather than retirement, Americans are saving for a "lifestyle choice" in their later years. According to the study, most households ages 55-64 do not consider retirement a near-term option. Four out of five have not stopped full-time work. Says Walters: "The goal is to have enough money for the lifestyle you want when you're older, not just quitting work."

Read next: Woulda, Coulda, Shoulda: What You Can Learn From the Top 3 Pre-Retirement Mistakes