I think you've got a few options :
1) If your mother doesnt mind, then you could just enter her age, so shes theoretically the account holder who then permits you to use her account.
2) If your mother is happy for you to lie because she doesnt want to enter her card details, just lie and its effectively the same since you have parental approval.
3) If you mother doesnt really know or doesnt approve, just lie and dont ask in future
On a side note this whole thread topic took me back to when I was 11-13 and playing the 1st 3 games in the Leisure Suit Larry series. They had a multiple choice question system you had to get past to prove you were an adult. Questions on politics etc from 20+ years back. It became a game within a game where me and my neighbour would hunt down the answers through encyclopedias
Truth be told i think half the inuendos and jokes within Larry flew over our heads, but we still had alot of fun and at least part of it was the forbidden fruit aspect.