13 based guys you know
like 12 go away

The problem is these two don't even LOOK the actual age they were (about 15). They look quite a bit older. If you really want to make this behavior look creepy to adults, you would cast girls who actually LOOKED thirteen. If you want to make this behavior look "sexy" to actual impressionable teens, you'd cast these two. Hmmmm.
You're right that girls mature at different ages. Traci Lords at 15 or 16 was able to pass herself off as a young adult porn star, but there is NO WAY she would have been a believable as a 13-year-old. Same problem with Reed and Wood.
"Let be be finale of seem/ The only emperor is the Emperor of Ice Cream"

You
do
know that no one in Hollywood plays their exact age, right? Dumbass

If she was born in 1987 and this was released in 2003, it's safe to say she was 15 at the time of release. But movies are typically released 3-6 months after shooting, so depending on her birthday and everything she may have been 14. I don't think this is that much of a stretch. See the above comments for an explanation on the ways biology, lifestyle, makeup, and clothing can affect how old a person looks.

I actually have a 13 year old neighbor who lives with her mother who kind of reminds me of the mother in this movie. She looked like a kid 2 years ago and now she could easily be 15 or 16. She already puts an makeup etc. I would say it's not only that some girls look physically older at that age by their upbringing makes a difference too. In the movie the mother is more this unconventional type.
In other families a 16 year old who is raised more sheltered might seem much younger.

Evan was born in 1987, this movie is 2003 so how was she only 14?
She was 15 at the youngest

Uhh, Evan Rachel Wood and Nikki Reed were 14 when they filmed the movie. People develop differently as they reach puberty. When I was 11, I was mistaken for being 15 because I looked slightly older and taller than my actual age.