I thought he was a decent enough, and believable, character.
I think he was clearly too young against Clegg and Truly, but I think they later aged him up a bit, though I never really noticed.
I'm fairly sure the character was, later at least, supposed to be the same age as Clegg and Truly.
The main problem with Billy Hardcastle is his character wasn't always well written; he was married, and often disappearing - probably to allow some screentime for Tom or Alvin. Sometimes it was a quartet with Tom or Alvin always around.
Keith Clifford never got third billing on a regular basis, so he never seemed like a true member of the trio.
I also couldn't understand in Clegg's birthday episode, Billy wasn't invited to the lunch, just Truly. This wouldn't have happened in the Compo years.
Worse still, when he left his absence wasn't explained.
Post Compo, all these Compo-like characters emerged, and it seemed for several years Roy Clarke was just experimenting with these characters.
Despite the age, it may have worked with Tom as a third man. It seemed that was the intention for his first few episodes. His character completely changed in every aspect over the next ten years. And Alvin seemed too much like Compo in some ways.
Billy Hardcastle seemed a good choice in the Compo type role to me - believable and funny.