Milk can legally be up to 48 hours old before it it’s picked up from the farm. With ever other day pickups being very common and multiple milking per day some get’s close to that. It can then sit at the factory for another 72 hours before pasteurization though this uncommon. It then varies though Ultra High Temperature (UHT) gives you up to 70 days at this point.

That’s the extreme which would be very unusual, but buying milk that’s ‘only’ 2 week old milk is reasonably common. All it takes is knowing how long the sell by date is after pasteurization and another ~1-2 days to get to that point and you can find how old this stuff is.