Martyn told the Liverpool Echo: "How long Tim can keep playing is down to his form and if he stays clear of injury.
"Also how he feels mentally is important as well.
"It is slightly easier for an English guy to stay in England and carry on playing.
"I don't know whether Tim would want to finish his career back home and play a couple of seasons in the MLS. But as long as he wants to do it and stays clear of injury then Tim can easily to play until he is 39 or 40.
"It is a season by season or even a six-month by six-month thing.
For the big stories and the banter follow us on Twitter: @tribalfootball