I've always liked the idea of older draftees having higher potential starting skills. 18yo max is respectable. Maybe 19 is strong, 20/21 is proficient. 22 is prominent. The age/max skill level ratio would have to be analyzed a bit to ensure balance. And I especially love the older the draftee, the lower the potential.
This would also help make the draft more productive for the top 1-6 teams. As it currently stands, it's highly unlikely to get a worthwhile draftee if you're one of the last picks. And that's not too realistic. I'm real life, you're not likely to get a franchise player, but you are likely to be able to draft a backup.