There are several categories of solider and staff in the Indian Army. Each of them have their own age, education, and marriage requirements. An additional complication is there are differing physical requirements for an applicant depending on which region of India he or she is from (but there are also allowances of a few inches or kgs if person is an acclaimed athlete, related to a current/former member of the Indian Army, and if the person comes from a chronically poor area).
Generally speaking, someone interested in becoming a soldier needs to be an Indian citizen between 17 and 23 years of age. Women have been allowed to serve in the Indian military for about 20 years, but are very limited on the positions available and level of advancement.