No! My teens are 15 and 13 and I wouldn't let them skydive. I don't think you should feel bad about it. Skydiving is an extreme sport and as such, there are risks associated with this. It's not like he's just asking to ride his skateboard! 18 seems like a good age to me...
Speak to the company that would be taking them and ask them what they recommend. Your son will want you to feel mean, so ignore that ;) But look into it further and find out how the company feels about it and if it's something they've done before.
My daughter is 16, I would let her go, but at 13 I'd need a whole lot more information before I decided.
No .. My daughter wanted to do one at 16 and I said no. She had a huge temper tantrum at the time. I told her it was my job to make sure she lived until she was18, after that it's up to her. Needless to say she finally did it for her 22nd birthday