You should have joined back when you were 13 and 3 months like I did
... While other people my age were out drinking/smoking on street corners I was in our ATC hut revising 'Principles of Flight' and finishing the night off with some drill.
Loved it! made some good friends and got some good flying time in
But you need to join before your 17 as said above. If you are already in the ATC and a Warrent Officer then I 'think' you can stay until your 18.
Andrew