The longer you let this lie stand, the more fragile the trust in your relationship becomes. Even if the guy isn’t going to care too much about a year’s difference, he’ll feel hurt or misled that you didn’t tell him sooner, especially since he’s already invested emotionally and socially with his friends thinking your age is one thing.
The best move is to be honest sooner rather than later. You don’t need a dramatic confession a light, casual way works: maybe joke about how you “just turned 19” and slide in the truth. Keep the tone calm and matter-of-fact. This shows you trust him enough to be honest now, and it gives him the chance to respond without feeling ambushed.
Honesty rebuilds trust, even after a small deception. Waiting makes it harder, because the lie grows with time and social exposure. If he cares about you, it’s unlikely to break the relationship but continuing the lie will almost certainly cause bigger problems later.