Same in the UK, but Steam uses credit cards for age verification there and refuses if you provide a debit card instead. Evidently the payment backends can tell credit and debit apart.

Yeah those are parallel systems for reasons that amount to technical debt.

It sometimes asks for my age for viewing a game and I can input any ol' date I want to. It doesn't even flinch if I input a different date every time.

I also don't recall them asking about my age when I was actually underage and paid using a PaySafeCard, but then again they didn't have porn on the platform at that point either.

They only enforce it in the "mature sexual content" category, which mainly applies to porn games. For everything else, including the "some sexual content" category, they still just take your word for it.