There is no way to determine the exact age of a sea turtle from its physical appearance other than to establish if it is a hatchling, juvenile or adult, depending on its size.
In the Indian Ocean, hawksbills and greens are smaller than their counterparts elsewhere. Good information on size at maturity does not exist for the northern Indian Ocean. It's tricky to tell visually if an animal is mature or not, especially females: they can be big enough but not actually be sexually mature.
After it’s death, the age of a turtle can be determined by a technique called “skeletochronology”, whereby the humerus (arm bone) is examined. These bones reveal growth rings that allow the turtle’s age to be calculated, much like we can calculate the age of a tree.
References:
Snover ML 2002. Growth and ontogeny of sea turtles using skeletochronology: methods, validation and application to conservation. PhD Thesis, Duke University, USA.