'At my stage of life, I think that the next couple of years will be the best of my career,' he said.
'If I have another three or four years left in me, I would like to retire at the top with Manchester United.
'I'm looking forward to adding to the medals I have already won with the club.
'And as far as I'm concerned, I still have a lot to achieve in football and that includes representing Wales in a major championship.'
He added on the club's website, www.manutd.com: 'After another four years, I would have thought it would be difficult for me to reproduce the form that I am capable of. It is only then I would start thinking about my future off the field.
'I certainly do not want to get to 40 and start thinking I quit too early. By the same token, you do not want to be struggling round the pitch at 37.'