Nowadays players maintain great level of fitness and it is not unusual to find 38-39 year olds playing international cricket. Along with Mohammad Hafeez, another player who maintains a good fitness standard is Shoaib Malik. For the batsmen and spinners, it is relatively easy. But that is not the case for the pace bowlers. Wear and tear from heavy workload means that most of them retire by the time they reach 35-36. A few who surprises me with their determination are James Anderson of England and Peter Siddle of Australia (although the latter no longer play international cricket).
Of course, a lot has to do with fitness and motivation/determination but another thing you would always see from cricketers playing in the later stages of their career is that they have an easy to follow and repeat action. I mean I cannot recall a bowler with smoother action than Anderson and for Siddle, it's not pretty but he has a good swift landing point as most of the time it's the leg injuries that pull the pacers back.
For batsmen, you will see the ones who maintain workload better will play for long periods. Talking about cricket, I see Ireland doing pretty well and I can only imagine how strong the team would look if they had their original player Eoin morgan in the team. If they had better batsmen and a experienced captain, which is what Eoin is best at, they would be a really good cricketing team.