Is it possible to have traits of mental retardation in middle school but grow out of it?

I know someone who at 12 was literally retarded. He had a special locker with no lock on it. He had a crazy imagination. He was really gullible, clumsy, and very odd. You would look at him and be like, "is he retarded or something? What's wrong with him." As we got older, he kind of grew up and appeared to grow out of this retarded state. At age 24 he appears to be a normal person. However, when I look at some pictures, his face reminds me a little of someone with down syndrome. It's like he partially has down syndrome or something. I really don't know and I'm curious. I'm convinced he has to have something. What can this guy have?