We don't use the'ing' form in this sense. I was playing basketball WHEN something else happened - yes. But Not together with 'AGE 15'. I played basketball at age 15 is correct. Do not use the other sentence - it is wrong. You could, however, say: I was ALREADY playing baseball at the age of 15.

Karen and Robert are right that "played" is the correct form, and you're right about the reason why you can't use "-ing.". "The age of 15" is too long to be a moment when you "were playing baseball." However, George is also correct. As Karen has said, it's correct to say "I was ALREADY playing baseball at the age of 15." Many native speakers (at least in America) will shorten this sentence by leaving out the word "already." The meaning does not change. If you say "I was playing baseball at the age of 15," you are emphasizing how young you were when you began playing baseball. It means, "By the time I was 15, I was ALREADY playing baseball." It's actually a very common construction, especially in phrases like "I was playing baseball before you were even born!" If you use it in this way, it's not wrong. But it does NOT mean the same thing as "I played baseball when I was 15."

I was playing baseball at the age of 15 when I broke my leg Notice one verb in past, broke, when verb in past progressive, was playing I played baseball at the age of 15. No other action

thank you! is it so? then I have to keep working on this point when reading English books to understand more about it or to be able to give a more specific question to the point.

"I was playing baseball at the age of 15." would be use for when you are tell someone that you still play basketball. Like "I started to paint when I was the age of 6 and now I am 25 and I still paint. "I played baseball at the age of 15." This would be used as when you tell someone that you use to play basketball. Like "Back in high school I was in the choir(singing group) but now I just sing in the shower" I hope this helps!