How many countries has the average American visited? And at what age do Americans usually travel the most?

I once met this American girl who had been to 9 countries and she was only 23 years old at the time. Is that considered an average number? And how would that number vary with age? How many countries would someone under 25 usually have been to?

Edit: I don’t necessarily mean travelled independently at your own expense. If your parents took you somewhere when you were younger, that counts too.