What I said was that it's not possible to verify someone's age with 100% certainty unless either you know their family, or you know for sure where they work / go to uni / what year in school they're in, or something similar
It is possible to verify someone's age, particularly with teenagers, when you can find out what school they're at and what year they're in. It's only an issue if you're going to have sex straight away, before you've a chance to find out.
What you actually said was:
It's a pretty extreme idea that all teenagers who have what is considered a normal sex life these days should just accept the fact that if they happen to be unlucky they could end up as a sex offender
But the problem is that would mean any of the millions of teenagers who engage in casual hook ups could potentially end up as a sex offender if they're unlucky.
Anyone, teen or adult could potentially end up as a sex offender if they don't take sufficient care. I've said before if they're not mature enough to grasp that they're not mature enough to be having sex. With sex comes responsibility. It's not a question of bad luck, but lack of care.
If teens learn to drive they have to obey the laws of the road. And if they don't they can end up with a driving offence.