staying the same yet growing up

this is relation to the tv version of bleach when Ichigo and co. first enter the SS they come across the soul of that bird that chad saves and he says that once you enter the SS you stay the same. but later in the show there is a flashback of captain hitsugaya when he is a boy. also there are often flashbacks to Renji and Rukia. this has led me to develop a theory that having spirit energy causes the soul to age.
I would like peoples thoghts on this theory if possible.

Actually they addressed this at one point. Apparently you do age in SS, but very slowly. People being hundreds of years old isn't so rare.