I suppose it depends on the ages of the other performers. I played the 7-year-old in a production of "The Sound of Music"; when I was given the part I was 11, and by the time we performed it, I was 12. I was very short though and could pass for a lot younger! So sometimes, if you look a lot younger than you really are, you could get away with being cast as a younger character.
There are also some shows where it's traditional for people to play characters much younger than themselves, even if they don't look anywhere near that age. For example, it's pretty common for the musical "A Slice of Saturday Night" to be performed by adults over 25, despite the characters being 17-year-olds. "You're a Good Man, Charlie Brown" takes this to a new extreme, where adults play the roles of kids.
Here's something you might find interesting:
link
This page contains examples of media that has used older actors to play younger characters. As you can see, it's a fairly common thing, so if you can pass for a 10-year-old, who knows, maybe you can do it! :)
