In my opinion, I think 13 is a good age to start. The better question is, what style of makeup should they wear. When you're in your younger teens, the natural look is better. 16 is a good time to hit noticeable makeup. But basically, it comes down with the personality, confidence, and what about her face she would like to enhance or improve on.
There's no specific age you should start wearing makeup. It's usually best to just... Not. It's not always good for your skin to go slathering various creams and pastes on it, after all. But meh. I'm 19 and still don't generally wear make up and only wear lipstick when I do, but most girls I knew were wearing make up from like 14.
I started wearing makeup at age 12. It's really early but I felt like wearing makeup then so I did, plus all my friends were too. So I don't know, wear it when you feel ready? I mean my best friend still doesn't wear makeup and she's 18