Ds was pretty bald until 18 months or so... well, really, he's 26 months and still doesn't have what I would consider to have "real" hair.
I, on the other hand, had NO hair until after 2 years of age (now my hair is ridiculously thick) and actually had a cousin who still had very little hair when she was 3. I guess the moral of the story is that you just never know.... just like everything else, kids get to things at different time