Sixty? That’s young – for a rich person.

Six of the 25 richest Americans are over 80, according to the Bloomberg Billionaires Index.

Carl Icahn and Charles Koch, for example, are 81. Earlier this month, Sheldon Adelson turned 84 and George Soros hit 87. Warren Buffett, the fourth-richest person in the world, with a net worth of $77 billion, celebrates his 87th birthday next week.

New data from the Internal Revenue Service show just how old the top millionaires and billionaires in the U.S. are. While people over 80 make up only 3.7 percent of the population, the IRS estimates they control a larger share of the nation’s top fortunes than people under 50.

The wealthy have probably always been older than the general population. Despite the example of Mark Zuckerberg, the world’s fifth-richest person at 33, it usually takes a long time to amass great wealth. Still, the imbalance is striking.

Every few years, the IRS analyzes the wealth of the richest Americans. Its Personal Wealth Study examines a rarefied group: people potentially subject to the estate tax, which is levied on individual fortunes of $5.5 million or more.

The latest study, released this month and estimating personal wealth in 2013, finds 584,000 Americans, or about 0.2 percent of the U.S. population, with a combined net worth of $6.9 trillion. People in their 80s and 90s control $1.2 trillion of that wealth. Adults under 50, roughly 43 percent of the population , hold barely $1 trillion.

Wealthy people in their 80s have the highest average net worth of any age bracket. One reason is that octogenarians have less than half the debt, as a percentage of their total assets, than adults under 60.

Measuring the wealth of the world’s richest people is difficult. Billionaires rarely take surveys, and they have every incentive under tax laws to make their fortunes look smaller than they are.