Very nice. There is really no way of telling a torts age, even by size. There are so many variables that could play in growth.
Gorgeous specimen. Manouria can very difficult to asses the age. If in the proper environment they can grow quite rapid. But tend to still take a while to reach sexual maturity. I'd say your animal(and this is only a guess) is somewhere between 5-8 years old. Also will depend on sex. Males stay much smaller than the females. Emys and all Manouria can live well beyond 80 years indeed. Especially given the proper environment and food schedule.
Woww..do you a picture of your tort?Very nice-looking tortoise. I have one about that size and it's 15 years old.