Yeah, I had one of those myself when I was under 18 too, I think it was called Maestro or something similar. However, it didn't work like a normal credit card (which I think only 18+ can have), platforms were clearly able to reject it, as most things I wanted to buy online didn't work at the time with it (this was early 2000s though), only with my mom's debit card.
Probably the same is true for those cards you linked, they're special "youth" cards that platforms could in theory block? Then requiring credit card "donation" of $N would still basically act as a age verification. I think debit cards might in general be available to people under 18, so filtering to only allow credit cards sounds like a start at least.
Newgrounds literally employed the same strategy for automatically validating a bunch of users, from https://www.newgrounds.com/bbs/topic/1548205:
> 2. If your account ever bought Supporter status with a credit card and we can confirm that with the payment processor, we will assume you are over 18 because you need to be 18 in the UK to have a credit card.
Basically, filter by the card type, assume credit = 18+, any other might be under 18.
> One mitigation around that could be the gifting of accounts
Yeah, referrals ala Lobste.rs. I feel like they get lots of stuff right, from transparent moderation to trying to keep it small but high-quality. The judge is still out on if they got it right or not :)