Even if you are 13, the vast majority of these bitcoin casino websites don't require age verification. In my experience, I've only encountered verification with Bovada/Bodog, and they aren't really 'bitcoin casinos' but they do take bitcoin...
but even if there is restriction, you can just put fake info, how they can know about your identity especially if you use bitcoin
actually even with fiat there is no way, you can ask your parent to use their credit card and that's it