This question is asked multiple times every day. The answer is always the same: for items requiring age verification, eBay only accepts payment by credit card.
Yes, there are other ways in which age could be verified, but they would require checking by eBay. Credit card holders must be aged 18 or over, so age is automatically verified. Which is presumably simpler for eBay.
If you think eBay should accept other methods of verification, please complain to eBay - not us. We are only other users like yourself.