When I visit my parents, I feel free to help myself to a glass of something from the fridge or fix a cup of tea or coffee if I feel so inclined. I do not take alcohol unless it's offered.

But when my parents visit their parents, they don't seem to have fridge privileges like that. They neither ask for beverages, nor take them if offered; nor have I seen them help themselves, even though both my grandmothers still live in the houses where my respective parents grew up.

But I feel free to help myself to something from the fridge chez one grandmother (although not to fix a cup of tea), and to ask if I'm inclined chez the other.

And when my parents visit me, they do on occasion ask for or accept something to drink, although they don't help themselves (mostly due to my personal boundary setting).

When in the home of a close friend who lives with their parents, I'll help myself to a drink from the fridge with a bit of an announcement ("I'm taking a Coke, okay?") if the parents aren't there, but wait for an offer or ask my friend if the parents are there. At the home of a non-close friend, or a close friend who does not live with their parents, I will wait for an offer. The parent differential is because parents have infinite money with which to stock their fridge full of an enticing variety of soft drinks, while students do not.