incorrect, growth continues well into the 20s, especially for boys. so delaying surgery as long as possible is a plus as you'll have a longer torso and more lung capacity. fusing too young, then you run the risk of rods breaking later in life. i had this happen to me and i was fused at age 11/12. rods broke when i was 18/19yrs old and refused at age 21/22.
honestly, i havent noticed much flexibility differences before and after surgery. i think this fear is often overplayed. i am fully fused from T1 - L5. partial fusions carry a huge risk of issues in late adulthood as there is more strain on the levels above the fusion causing degenerative disk disease. alternatives are flexible fusions but i dunno of how prevalent they are in kids, you mostly see these in adults who can walk.