It comes down to not being an idiot. Behave like it's a real firearm. Don't think about how mature you are or how ok with it your parents are... think about what kind of reaction other people will have if they see a 15 year old with a gun. Joe Public doesn't like that sort of thing. And I guarantee you "oh, it might be a toy" will not in any way enter their minds.
While I don't personally support <18 owning an airsoft gun, as long as you're not a selfish idiot who is actually capable of comprehending what other people's reactions will be if they see you with it, and act appropriately, you should be fine.
PS. All bylaw stuff applies, so it may actually be illegal for you to discharge an airgun in your municipality.
"Someone in a Prius tried to race me at a stop sign the other day. I couldn't believe it. I had him for the first 100 feet or so but I can only walk so fast."