Are both sentences correct?
1.He has collected teddy bears for 12 years.He started to collect them when he was six.
2.He has collected teddy bears for 12 years.He started to collect them since he was six.
In both sentences1 and 2, do U have to use perfect tense for the main clause in the second sentence.If so, is it correct to use either present or past perfect?

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It's better to use Present Perfect Progressive in the first sentence as the action is still in process and Jerund (verb + ing) after the verb "start":

He has been collecting teddy bears for 12 years. He started collecting them at the age of six.

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I agree

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Besides, I think that
"He has collected teddy bears for 12 years." 
is not very nice

"He has collected teddy bears for 12 years since childhood" is better.

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He started to collect them when he was six. 
BUT
He has started to collect them since he was six.