Is it legal for a minor to go topless?

It's legal pretty much anywhere here in the UK. I have a hard time imagining any kind of legal system where having breasts was a criminal offence.

If topless sunbathing is allowed then the problem with say a 16 year old girl sunbathing topless on a beach is that the police would never go near her to ask the question, check her ID. 15/16 year old girls sunbath topless on Miami Beach all the time, the Beach Patrol doesn't get anywhere near them unless they have alcohol out in front of everybody. If topless sunbathing on a public beach is illegal then that is a whole different issue.

It depends on the beach. If it's a nude beach, most likely, if you're with a parent. If it's a regular public beach, no.

depends on gender-der

I don't think so, but might as well enjoy all those mexican pedophiles while you are it too.

If the minor is about 1 - 2 years old, yes. Any older than that, it would be considered indecent exposure. A guy is not a woman and women are supposed to behave like ladies.