India is a huge and diverse nation that faces many challenges as it continues to modernize. An older Prime Minister can bring years of practical experience in industry and politics to office to help it through these tumultuous changes. On the other hand, a young Prime Minister would not be entirely bad, perhaps open to new ideas and bringing new ways of thinking into the office to accomplish some of the same goals.
Although an older Prime Minister can do a very good job at running India, being older does not intrinsically make them better. Age has very little to do with how good someone is at running a country. A younger person can do just as good of a job, if not better, at running a country.