I thought that, by age 10, you were a grown-up. I think I got this idea from babysitters who were only in their teens, but were in charge.
A childhood belief about getting older

others believed things like this
- I thought the 'dubbing mixer' on TV credits was the person in charge of stirring the make up for the actors.— Clive
- I thought grown ups were born grown ups.— mea
- I used to believe that grown-ups were born "grown up" and that I would stay a little kid forever.— Still A Kid
- We knew a woman named Barbara Hannah, and I thought she was was in charge of Hannah-Barbara!Anon
- When I was a kid, I thought the musical Mamma Mia did songs from the A*Teens (the teenage version of Abba)Anon
- I used to believe that grown-ups were born grown up.— Juleen