Answer by Commander Rathore: I am going to write on this issue mostly about officers and touch upon the same about soldiers in the end.
The earliest an Indian can become an officer in Armed forces is through NDA. Earliest a cadet can join NDA is at 17.5 years of age. He completes his training in NDA (3 years) and IMA (1 year). By the time he is sent to an Infantry unit he is 21.5 years of age. By this time, a young lieutenant has physical stamina, strength, weapon firing practice and confidence in his ability for combat.
War is, naturally, extremely strenuous. It requires operating under bad weather, little or no sleep, carrying huge loads, running etc. It is not a job for the middle aged or the old.
Creating a soldier requires a significant amount of deep psychological training along with physical training. Officers & soldiers must be trained to kill, to work in conditions where your friends and fellow soldiers are being killed. It is much easier to condition young people with little or no life experience than it is to train the older people. In addition, wounded 20-year-old will recover faster than a wounded 30-35-year-old and they also have the potential for a lengthier term of service. Anyway, as an officer keeps growing in age, he begins to command larger group of junior officers who in turn command soldiers. The soldiers also join at the age of 19 to 22 years of age. Thus, immediately after training they are suitable to fight a battle.
In case of an armoured or artillery officer, the junior officer does not get to command a tank or guns being fired by his soldiers till he qualifies under a senior officer.