If you have sex at an early age will you have trouble getting pregnant later on?

Possibly. Teenage sex is fraught with chlamydia, gonorrhea, herpes genitalis, hpv/genital warts and cervical dysplasia/cancer, and other sexually transmitted disorders. In that sense, yes it can adversely impact your fertility and even life/health later on.
Hard to say. Sex at an early age often predisposes for stds, and that can impact later fertility, but strictly speaking, the age of first intercourse does not have much else to deal with infertility. It does, however, predispose you to becoming pregnant at a younger age, as many young people do not use proper birth control.