"People asking me about retirement is not new and you just get used to it," he told the Manchester Evening News.
"I understand that. It is part and parcel of getting older in sport.
"I remember someone saying that Moscow in 2008 would have been the ideal time to retire. They were not saying it in a nasty way but they were just suggesting it would have been a perfect time to go.
"That was five years ago - I would have missed out on two further Champions League finals as well as winning three league titles.
"I like to think I have contributed in those great times since. There is an ageism thing in sport but I just take it as a challenge. I will know myself when it is time."
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