Depends on thier lifestyle really.
anywhere from 10-13 is about average. 15 is above average.
I know our oldest labs sire lived strong untill he was 15 so did many other dogs in his pedigree.
I would seriously consider a diet for your boy. 5-9 is thier prime..5 comming into prime and 9 going out of prime if they lead healthy lives. Having your boy overwieght will have major downsides as his age progresses. Hip problems is only one of many issues that can arise having a overwieght lab.
Plenty of exercise and have his thyroid tested and a full blood panel work up to see if there are any underlying issues that may be contributing to his being overwieght.
The best exercise if you are already worried about his hips is swimming. Yes labs love to swim even if its cold! Thats what they were bred to do. Keep the exercise in short sessions. Start out with 10 min and slowly work up to 15-20. I wouldnt advise anymore than that in really cold water. Do this daily and you will have a happy healthy boy!!
THis lab is 8 and still winning trials.
http://www.creekrobber.com/
This lab is 12 and still going strong.
This lab is one of mine who is 8 almost 9 He still hunts all day.