I felt as though my TB finally stopped growing and started filling out at age 7. Not terribly uncommon to have a later growth spurt. Some are slower than others but I can't imagine him growing much after age 6/7.
My then-5 y/o TB measured a touch over 16 hands in Feb '12. He now is still a few months from truly being 6 yrs old, and is a solid 16.2. Hasn't filled out much yet, although I've noticed his chest is wider and beefier than it was in last spring/summer.