Not if he was playing every week, no. Playing 1 game out of every 3 however or coming on as an impact sub could be very different. He's only 36, which is a fair bit younger than Ryan Giggs or Paul Scholes were when they were still in the squad when Manchester United won their last Premier League...
I would have thought if Liverpool are in the title hunt still in January, Gerrard on something like a pay-as-you-play basis with some kind of future coaching role at the club lined up would be a possibility. Gerrard's about the same age as Gary McAllister when they signed him under Gerard...
Possibly because they enjoy playing football and they know that whatever they do next will never replace the buzz of putting their boots on and playing on a Saturday/Sunday afternoon and so they want to keep playing while they still can.