Question

If an individual does not develop at the same rate as their peers, they may be developing abnormally

Answer 1

"abnormal" means "not normal" - so it can mean "not standard" -therefore if someone is developing at a different rate than most of the people you could say that they are not developing in a standard way - so you could say that they are developing abnormally. The sentence is correct, although it might not be the best word to use.