The consensus among the ortho community is that once the growth suture in the palate is "fused" then growth is not possible without surgery.
HOWEVER
Practitioners have experimented with this theory, and found it to be false. Although slower, the palate can be grown and moved forward well into adulthood.
This is according to the work of Dr. Kent Lauson DDS., MS and his book "straight talk about crooked teeth: The new orthodontics"
Ive spoken to 2 orthos in my area who have confirmed his theories have worked on a number of patients in practice.