Well, I think it comes down to common sense. When puberty hit, our body changes to accommodate for sex, and that is also one reason why our hormones go crazy and we become interested in the opposite sex.
Now if having sex at a young age (but after puberty) was actually harmful in the long run, why would our bodies encourage us to go for sex? It doesn't make much sense to me. No, I think listening to our own bodies signals is more accurate than some random doctor, at least in this case.
