What are the risks of a 15 year old being pregnant and giving birth, vs. A 25 year old?

A smaller pelvis. A 15 year old generally is still developing the space between her pelvic bones for adequate vaginal delviery. That pelvic "widening" is most often complete by age 16 of development and therefore there are fewer c/s's in adult women. The other main risk at 15 is poor nutrition resulting in more intra uterine growth retarded(iugr) babies.