Strictly, whether grammatically or semantically, the correct answer to the question "If you could be any age, what age would you be?" is a lone number, as “12” or “I would be 12”
Strictly, it’s a conditional question only because your forced it to be. Please explain the difference between what you asked and “what age would you prefer/choose/want/hope/ to be?”
“To set up the condition” one would first need to have been required to set up a condition, which one was not.
“I would be 12” is a complete answer to the question you asked.
“I would be 12 because I couldn't play then” is understandable, but not necessary.
“I would be 12 (again) because when I was 12, I couldn't play” is probably what you meant. Is the difference obvious?
“If I could be 12, I would play more because I couldn't play when I was younger” does not need - in fact, suffers from having - the “more”
“If I could be 12, I would play, as I couldn't when I was younger” is better. In either case, ““If I could be 12” does not answer the question as “I would be 12” does.
All of your conditions are perfectly reasonable in and of themselves but grammatically, none of them is in any way relevant to the question.