She will never be too old to learn new things. My horse, Stoeka, is 25 years old, and she never knew anything about turn on the forehand or lateral work. But I have successfully taught her, and now she does it like an old pro.
I'd say give it a try, but first you must learn all the odds and ends about neck reining. It's no use trying to teach a horse to do something you have no clue how to do yourself. ; )