He started -----to ride a bicycle when he was 3 years old.At

答案：He started 【learning】to ride a bicycle when he was 3 years old.
解析；start doing sth.开始做某事.【已经开始做了.】
start to do sth.开始去做某事.【还没有开始做.】
从when he was 3 years old.可以知道已经开始做了.
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