回答：提问句中的stated 应为started,全句应为How old were you when you first started playing the piano?
句中动词start(开始)后接另一动词作宾语时,其形式可用不定式to do,也可用动名词doing,提问中的句子started playing 就是用了第二种形式,即动名词形式.又如：
When I first started to learn/ learning English,I found it hard,and I nearly gave it up.
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