1. She's 17, which is the age of consent in some states 2. Wait til you get to the twist at the end...
She's 20 and Booker is 38
It's funny, Elizabeth looks relatively young when she emerges but gradually ages 10 years as the game reaches a conclusion.
I think it's just that her short hair makes her look older as does her dress. The dress is a big part as it was designed for an older woman of high social standing. Also Elizabeth is much more emotionally mature as the game progresses which makes her seam older then she is.
For me the last point is the biggest as I've met people I thought were late 30s, but were actually in there mid 20 because they acted much more mature then there age. The opposite is also true I've met some people well into their 40s that still act like they're 21.
Elizabeth also starts the game with a very child like personality and her dress and hair are in a style more common amongst young girls.
I thought Booker was mid 40s as he fought at wounded knee, but I forgot that in that time men could enroll at a younger age.
"What is better. To be born good or to overcome your evil nature through great effort." - Paarthurnax Gt: Ghosta2