Teenagers of the same age having sex is commonplace as low as 12 (more likely 14) where I live and it's never bothered me or any of the people I know who've done it as far as I'm aware

The big issue is teen pregancy and consent, so, ensuring access to condoms while all the religious people keep crying about it and touching them a word or two about the seriousness of consent

Consent about pictures as well

Teen pregancy is probably the reason why we don't want to normalize it, not the issues you would get from predatory relationships (developmental issues, warped views of sex, social integration issues, ptsd, ...)

Teen development is compatible with sex as long as it comes up normally

(The place is Tahiti)