"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.
I think it takes 8 year to end the Revolutionary War. It start in <span>April 19, 1775 and end in September 3, 1783. ( Hope my answer would help you. )</span>