I was the tiny kid at school, and when I went to university people thought I was a child genius, and that I was really about 15. I only got my growth spurt between 19 and 20, and am now at the heady height of 5'10".
I'm 26 now, and it was remarked only very recently by my older friends that my body seems to have just stopped changing. They mentioned last week that I seem to have finally filled out (not got fat!) and reached that final finished article. Everyone goes at different speeds!
In response to the question, your best indicator is your parents, but even that can be misleading sometimes