I think that if you're strongly autistic, you may not realize you're different. I didn't realize until the fifth grade primarily because I just kind of ignored other people and didn't have nor want any friends for a very long time. I was just oblivious, and even when I did realize I didn't really define myself by being different. But I'm in the minority. Most of the autistics I've talked to, including both Aspie and Autie types, knew they were different from quite early on.