I think by using past tense, it means that you were able to walk in the past but not in the present, so I think it's better if you say "I could or I learned to walk when I was less than a year old."
Nina, I think both "I could walk" and "I was able to walk" are acceptable. Both indicate ability and capacity to walk, if so is desired and attempted. Personally, I wouldn't prefer any of them.