To add, SIR looks at your parents. If you were formerly singaporeans or Singaporean PR, you are eligible for NS. This is to close the loophole of PR's who keep their kids in Singapore on student pass and then send them out of Singapore to study when they reach 18 so that they can escape their NS liability. The general rule is that if you want to come back to Singapore to work after that, you will have to serve NS.
FYI, the reservist laibility for Singaporean males is up to between 40 - 50 so you don't have to worry too much abt the being the oldest person there. Also there are regular medical check up's to ensure that you are medically fit to undergo the training.
Finally if you are a 2nd generation PR, you should seriously consider giving something back to Singapore and finish your NS liability.