Dr. Deb Bennett wrote a piece on this. http://www.equinestudies.org/ranger_..._2008_pdf1.pdf
"All horses of all breeds mature skeletally at the same rate."
No horses are mature before the age of 6. Add 6 months on for a male and if the horse is taller and has a longer neck it will be even longer for the lateral fusion to be complete. It may take a taller male horse of any breed up to 8 years to have fully mature bones.
Now - the article aside - I do know my short haflinger continued to fill out with his muscles until age 7. Mentally - I'm still seeing improvements in his mental maturity at age 8 and I suspect that will continue to improve over the next year or two.