I would tell them to strive to be the best that they can be..the HEALTHIEST that they can be. Eat right and exercise...the sooner you get on the exercise bandwagon the better. I did'nt discover fitness until much later in my life, and didn't get certified to teach fitness classes until the age of 38!! I wish I had taken better care of myself when i was younger, and didn't drink soda and all that other nasty crap that I did to my body.
Dont' try to look like anyone else....beautiful women come in all shapes and sizes. Accentuate your better points..don't STRESS over your problem areas. For years I hated my thighs to the point of wearing loose clothes all the time (this was before JLo made it okay to have a little bootay)....If I could have my 25 year old body again i'd be working it, not wearing a burlap sack every day.
LOVE YOURSELF...LOVE YOURSELF...Don't wait for a man to validate you.....LOVE YOURSELF!