DeletedUser34 Just to be precise for other readers, the source of Grayjay is open - you can audit, change and compile it on your own. But yes, it is not Open Source.
I kind of don't want to be direct, but what's age-restricted and behind login on Youtube, is very often just freely available on the alternatives. I didn't make sure, but I think Odysee, Rumble and PeerTube Android apps are Open Source. You could use those, but it's easier to use Grayjay, as it makes it transparent - you would not even notice you watch from them instead of Youtube.
The only other thing you could try, is to use a burner Google account, but if they would try to do proper age verification with ID or credit card, then I think it's outside the scope of this forum.