Well you have 2 options:
One, use fake ID and hope they don't notice.
Two, send real ID and hope they don't realise how long you have been a customer.
I thought about this for a minute and I think there is a better chance of them not realising how long you have been a customer, could be some newish guy/girl who works there and only needs proof you are overage, they might not even take into account when you joined, it could be a quick case of "is this guy legal age?" "yep" "ok, next", unless they have some sort of coding that alerts them that you have been a customer before you were of legal age, or unless they are specifically looking for this, also, IF they do catch you, I don't really see why they could keep your money? All the money you want to withdraw, did you earn this before you were legal age or after? If you won those bets while you were of legal age, then everything should be fine, they could probably choose to ban your account for "previously breaking the law" but I would imagine it's all good if you are withdrawing money you legally won.
Even someone who is working their 1st day at the company whose job it is to check the ID will know the in-depth routine for something this serious, so I would definately think 100 times before going with this plan.