It's legal at 16.
When I was 14 I wanted the top of my ear pierced. My mum said if I still wanted it at 16, then I could. Then, when I got to 16, I didn't want it, I wanted my eyebrow pierced. My mum said, when you're 18 you can get it done. When I was 18 I though "urgh, thats so tacky" and never had either. Instead I went and got a tattoo done, which at 25, surprise surprise, now looks tacky.
What I would say is that a belly piercing can be discreet (when you need it to be). If you ever get bored of it you can just take it out and let it heal over. No harm done. I'd say go for it (at 16, obviously). Good luck, and take care of it or it'll get infected ;o.