You can be certain about each other’s age.

...unless they have a fake ID. Which in turn is not in any way a defense should you perform certain acts with them that are illegal due to age. You're just supposed to be able to sense the difference between someone slightly above and slightly below the line in your jurisdiction.

Wait, regular young people didn't used to ask for ID on dates? I guess I was just paranoid.

The problem with this strategy is you can still be prosecuted for statutory rape even if you relied on a fake ID.

FYI this is not allowed to be used as a defense in court in some states (e.g. mine). If you card them and they have a fake ID, you're still criminally culpable for anything untoward.

Unforch, knowing someone's government name is disappointingly ineffective at preventing or remediating harm, in my experience.