I'm turning 25 in May!
I know I'm not really a reference since I constantly get confused for an 18 years old at best but once I did a costume where I had to look like a pre-teen and I guess my makeup was good because I was asked if I was 15~16 a lot. ^^;
So here's what I did:
I put on a pale thick foundation and a pale powder over it (I also have dark circles under my eyes so I need it).
I used a baby pink blush that I applied generously on the 'chubbiest' part of my cheeks (cheekbone?) when I smile. (I usually smile to my reflection to apply it ^^; )
I applied black eyeliner because it's all I had but I think a dark brown would work better. I put a rather thick line in a circular shape to make my eyes appear bigger.
Lastly, I put a tiny bit of eyeliner pen under my bottom lash line and some mascara.
I think the smile also influences a lot the look. If you have a more serious smile it changes your face. I usually try to go for a small impish smile without showing teeth.
Anyway! I hope it helps!!