This is quite a complex matter as it depends on the species and their life cycle respectively. For the western Palearctic there is a great book on that topic, but I do not know if there is something comparable for India.

Regards

Johannes

Gulls of Europe, North Africa, and the Middle East: An Identification Guide : Adriaens, Peter, Muusse, Mars, Dubois, Philippe J., Jiguet, Frédéric

Age determination in gulls can be challenging, but there are a few methods that ornithologists and birders use to estimate the age of a gull:

It is important to note that age determination in gulls can be difficult, and in many cases, it may not be possible to determine the exact age of an individual bird. However, by combining multiple methods and observing a variety of characteristics, experts can often make an educated guess about the age of a gull.