Rich kids usually have a pretty good idea that their families are wealthy. Maybe the Gulfstream tipped them off. Or the fourth home in Aspen. Or the Elton John performance at their 16th birthday "destination party" held at the Esperanza in Cabos.

But wealthy kids don't learn how wealthy their families are until—well, sometimes after their parents die.

A new study from U.S. Trust suggests that millionaire parents may want to start a little earlier. The study, part of U.S. Trust's Insights on Wealth and Worth series, found that very few parents believe their kids should know about the family's wealthy before the age of 25.

About half say the kids should be told when they're between 25 years old and 34 years old. About one fifth believe the "kids" should wait until they're at least 40 years old to learn the numbers.

The study found that more than half of the parents had disclosed "just a little" about their family wealth to their kids and eight percent have disclosed "nothing at all."