If you think you might go, the best time to tell people you're a non drinker is when you get the invite to drinks. The second best time is as soon as possible
If you're not intending to go to these things, 'definitely' is probably not the best response.
I'm at the point where I can go to these things if I want to and it's not a problem for me to be around drinkers. I'll bring something for myself to drink.
'I don't drink' isn't really that personal. Most people don;t care what I drink or not.
If the room falls silent, a glass breaks and someone gasps when you say that, it's probably not the crowd for you
D