Definitely think work and family are the big things. I'm only 28 and physically I should be able to continue.
The difference for me was when after a hard day work you get to senior training and there's limited enjoyment. The last thing I need after hard day work is to be shouted at for dropping a ball in a drill.
At this stage I decided to take a step back and train on my own during the week. It finished me as a senior player but I am still playing intermediate and Junior. I will most likely play junior football until I cant move but I also see myself moving into some form of management.
At the end of the day football is not the most important thing in life. Your own personal health and happiness is whats important.
When you are exhausted after the warm up...... When your time is better spent coaching your children's teams..... When having a hurl broke across you loses its appeal.... When you care about your injured opponents on the ground..... When you say "its only a game".....