Starting later thanks to books and your Mom,

Books do good things to everybody!

Although early (go, define!) sex is nothing bad, in case you think it has to be avoided, here are your answers. And, no, it is not socio-economic class of the family or race/ethnic origin (we are talking US data here, keep in mind).

Two factors play critical roles in protecting girls-regardless of their socioeconomic status and household structure-against early sexual activity: (1) the quality of their relationship with their mothers and (2) achievement in school, specifically their reading proficiency.

(The research from Girls.inc + the commentary from Feministing.org)

So, education and social networks are the key! We can still look more into some more particular variables and such, but you get the general image.
And knowing that the academic success is also very much household attitudes-bound, more support for education (oh, books! oh, good public education!) and quality family time (oh, parental leaves! oh, work that's compatible with private life!) should be the key for anybody that actually feels like supporting family values (!) and postponing sexual activities.

If that's not your cup of tea, this might be,