Each child and each party is different.
DD is 17 but the youngest in her year so some of her friends are 18 and able to buy alcohol so when she is going to a party (which is probably every couple of months) and asks me to buy her something I do, because otherwise her friends will just buy it so doesnt make any difference!
Also means that if she is in trouble or unwell I know she will ring me rather than worrying about getting into trouble like I did and staying in an unsafe situation just so I dont shout at her.
DS1 is 19 and has never drank, he doesnt even like fizzy soft drinks.
FIL was an alcoholic so we are aware of the downsides to drinking. We don't drink that much - I probably go 'out' drinking about 4 times a year! In the house we have the odd glass of wine on the weekend but rarely finish the bottle. So I think we model responsible drinking and knowing your limits.
We are lucky that DD friends are a good bunch - they always make sure everyone is walked home so no-one is alone and I often act as taxi too. She had a halloween party. I only provided her with drink for herself but turned a blind eye to others bringing stuff - she had about 30 people and we were upstairs so knew if anything kicked off we could take charge. She stuck to the party end time, and some stayed to help clear up. I guess if your child had a group that caused fights, and left a trail of destruction it might be different!
Comparing it to drugs or other completely illegal activities is not really a goof comparison - I am not taking drugs and never have so feel justified taking a hard stance on that whereas I would rather they eased into drinking slowly in more contained safe environments (house parties where parents are banished upstairs) than go out for the first time clubbing and not know their limits.