Yeah, his age is a little weird. From Wikipedia:

In the very early days of the strip, Charlie Brown was explicitly identified as being four years old; he would age very slowly over the next several years, being old enough to attend elementary school by the 1960s

Schulz probably did that because you get a lot more material if they can go to school and play baseball and whatnot. Also even in the 1950's, kind of weird to have 4 year olds running around without any adults in sight