When you're thinking about instruments as objects - your age example. How about this:Rarely do I see "He played a piano" but I guess this can be OK when I say, "Last night, he played an old piano but the sound was still excellent," "I have played a lot of pianos but this is the best one."
3, Let me add my questions: When the following phrase can be correct and natural?
"play a piano"
I have always played grand pianos, but last night I played an upright piano for the first time
Retired English Teacher