Probably easier for me, due to age (51) but I'd just say, "I'm not drinking", and if they pushed the issue, I'd ask them, "what part of I'm not drinking do you NOT understand"? Again, probably an age thing, but if you can't give me a concrete, tangible reason why my actions or non actions are causing you harm in some way - F ya!
Live and let live, they want to drink, but you're not asking them to not drink, how dare they ask you to drink if you don't want to?