Puppy or not?

Firstly I would like to thank you for a very informative site! I rescued my 18mth JRT a few months ago now and he is fitting in to our family very well. My 3yr old son adores him. I would however, like to know how long do JRT stay puppies for? He is quite good on the lead and slowly becoming obedient. I can still see that silly, cheeky puppy stage in him (mind you, its often quite cute), and am wondering how long this will last?