It is certainly quite common in the north of England where I am from. The female equivalent here is lass. Up in Scotland laddie and lassie are also quite common.

Its use is more common in the north of England, it’s used in the south but would be more as a friendly reference to a young person, i.e. an older person referring to a younger male e.g. “look at that young lad over there...” “there was a group of young lads...”

"lad" is used in North America too! Unlike the UK, we use it to jokingly pretend that we have a hyper formal relationship with someone. Ex: if you were to say "he's a good lad" in Canada it would be a compliment, but it's also kind of like an inside joke. Here the word "lad" is used to imply that we have a formal relationship with the person, when they're actually just our good buddy. :)

Yes it is, in the UK anyway.

It is also used amongst young people in Australia to describe a young man. In Australia, it is considered fairly informal but it's not disrespectful.