It's a security measure eBay require a credit card on file and for now, this is the only method of proof of age eBay accept, being that you have to be 18 years or older to own one.
You may have been a member for 24 years plus, but this doesn't prevent a younger member of the family using your account to buy, so some members do prefer not to allow a c.c. on file to prevent this happening.
@moppetjapes