what is the most appropriate way to ask this question How old were you when you were able to sing? How old were you when you could sing? and what is the reason? Thank you,,,
Neither of those work very well, because, presumably, a person becomes able to sing and then retains that ability for some time. Thus, the way you've phrased it, the answer could be any time between when the person first learned to sing, and now (or whenever they stopped singing, if applicable).
It would be more appropriate to ask: "How old were you when you first learned how to sing?" Or "How old were you when you first became proficient at singing?"
This is because you're more interested in knowing the earliest point in time at which they could sing.