And yet not one single person has explained how it's possible to be 100% sure. Perhaps you could suggest a foolproof method?
That's the whole point, but you know that anyway. Clearly it's your view that men should not be expected to show caution or reserve when it comes to 'hook up' sex because it's acceptable collateral damage for the occasional 12 yr old to be raped so they can indulge in high risk sexual exploits without worrying about whether the girl/women they hook up with is actually above the age of consent, never mind they could even be younger than 13.
High risk sexual endeavour = risk of STDs, unplanned pregnancy, physical danger from someone you don't know (assault/robbery/worse) and the possibility the person you hook up with is actually a child not old enough to consent & you end up a convicted rapist. Known risks, particularly in light of this case.
Anyone who wants to indulge in high risk sexual exploits should at the very least be capable of knowing and understanding the implications of the risks involved, accepting any one of those outcomes is a possibility. The ' but she looked older' excuse is wearing a bit thin IMO.