Once they went away to college, any time they came back they could drink*. Before that, they had to ask to drink from our glasses.
Although once the oldest had been to college when he came back he went through a phase of making alcoholic milkshakes and the high schooler was allowed to drink those.
*We are not a family that drinks on a nightly basis. If we are hosting a party there are drinks. If we have friends over there will probably be wine. My two college kids knew if they wanted to drink they were not going to drive that night. They could have friends over but couldn't go out. And no, we did not serve their 20 yr old friends alcohol unless the parents had also come over and they said it was also okay.