Husband and wife stay out until 3 AM, are accosted by mother-in-law who was watching their 12 and 16-year-old: 'I don’t like [...] being treated like a child sneaking in'

Do you lose the right to stay out late when you have kids?

I sure hope not. Being a parent is hard enough; you don't also need to feel like a kid while you're doing it. It's kind of the unspoken rule of culture that once you have your own family, you get to make your own rules. You might still feel responsible for those who came before you, but you don't necessarily have to follow their rules. You can raise your kids however you'd like; you don't have to enforce the same rules in your house. Plus, you yourself don't need to live by them. If you want to eat ice cream for dinner, that is your prerogative. You might not want to because you know it would make you feel icky for the rest of the day, but you could do it if you wanted to. That's the beauty of being an adult. 

When it stops being fun is when someone tries to harsh your vibe about your own choices. Maybe you've had a friend like that, who is constantly judging your life choices. They probably weren't your friend for long. You're allowed to stay out all night and party til dawn if you'd like to. If you have kids, you have to make sure they're well-taken care of, but these are not mutually exclusive experiences. Parents need to have a little fun once in a while, too. It's important stuff.