According to Dr. Deb Bennett who specializes in this type of research, a horse does not fully mature until 6-8 years of age. A very small mare may be fully mature as of 5 1/2 years of age, but the taller the horse, the longer it will take for maturity, and males take 6 months longer than females.
I know my haflinger gelding was physically widening out and get a more mature last year as a 6 year old. This year as a 7 year old I finally am not getting much change from him.
So yes, I would expect a 5 year old to still be filling out some for at least another year, if not 2 or 3.