Congratulations. Your parents can now legally charge you rent. And yes, that selective service thing is a MUST.
But there are other milestone ages you can look forward to:
21 - The year that everything you couldn't do, even as an 18-plus, you can now do.
25 - The year that your insurance rates *should* go down
31 - The first year ones free themselves from Selective Service. Of course, by this time, you may very well be halfway towards military retirement.
55 - You can join AARP. And all sorts of other programs aimed at "relics".