Lisa
Hi Marcy, The reason he won't sit up or walk until such a late age is cause to sit...he has be able to balance himself including his head. Which some learn to do sooner then the doctors think. My daughter didn't sit until she was 14 months. As for walking...they have to learn to balance on shorter legs and smaller feet the other children. It all basically depends on if your child has any other medical problems and if he's ready to do it yet. Children with Achondroplasia are just like other children. They do things at different ages. Not every child is the same. Good Luck, Lisa