I'm doubtful as to whether these two sentences are correct.
I wasn't able to ride a bike until I was 20.
I was able to ride a bike until I was 20.
My suspicion is that only the first sentence is correct, but I don't know why the second should be wrong. However, I feel that the second will be better with past perfect in place of simple past:
I had been able to ride a bike until I was 20.
Best Answer
All three of your sentences are grammatically well-formed, but they mean different things.
I wasn't able to ride a bike until I was 20 means that you learned how to ride a bike when you were 20—before that you could not ride a bike.
I was able to ride a bike until I was 20 means that before you were 20 you were able to ride a bike but at that age something happened—perhaps you lost a leg!—after which you were no longer able to ride a bike.
I had been able to ride a bike until I was 20 describes the same sequence of events of facts as 2., but is used when your current topic is a past situation—some time before the present but after your 20th year. For instance:
The preposition until designates a timespan lasting up to the timepoint named by its object, and ending at that point.