What?
Don't you think you're a little old to be watching My Little Pony?
Depends on the person.
Most women aren't interested in their boyfriends watching kids cartoons. Hypothetically speaking, if your girlfriend liked the fact that you watched my little pony then you probably wouldn't be getting much, or she just wants to be your friend.
To what exact age?
Your own age, if you're between the ages of 1-11 then it's understandable.