You were up until

[tufguy]

If someone sleeps at three in the night. Is it ok to say to that person "You were up until very late last night?".

To a shopkeeper "You were open until very late". Is it fine?

 

[Raymott]

"If someone goes to sleep at three in the morning ..." This is natural. Yes can say , "You were up until very late last night." No question mark in the quote.
Yes, you can say that to a shopkeeper.

 

[Tdol]

You could also say:

You were up very late last night.