Several is a word that is used to indicate 3 or more. IF you are 12 and you've been doing it for several years then that means you would have started it around... 8 or 9 years of age. I'm not one to judge if that is indeed the case, but I think you're probably exaggerating a bit on your experience. IF you had proper training, then why are you asking us? Why not ask the person who properly trained you?
Here's the thing. Maturity or not, your body is growing. It's going through changes, and it will continue to do so for several years to come. I'm not a physician so I can't say for sure what all will change inside your head between now and maturity, but I'd hedge a safe bet and say not to do it until you're an adult. Further, speaking as an adult, I'm not comfortable with the idea of teaching or advising a minor as young as you on anything that involves dangerous stunts, driving spikes into your skull, etc, etc. As Randy has already pointed out, it's a legal issue. I don't want to be responsible for your injury (and god pray that it never happens) should something go wrong.
Also, Aaron has a point about the performance aspect. Unless you're from a Carny family, where other adults in the show whom you are close to is doing similar stunts, you just randomly pulling out a nail and hammer amid your card tricks and sponge balls is going to look odd. As an adult watching a 12 year old do this I'd be concerned. I just don't know how you could pull it off right... (and I'm making sweeping general assumptions here that may be highly inaccurate) I don't think it would fit a 12 year old's character, or thematically work within the average 12 year old's show line up. There is no justification for doing it- other than to get a visceral reaction from the audience, and that isn't always a good thing.
IF you've been trained proper as you claim, ask them. Otherwise my advice is to stick to card tricks for a few more years to come.