Why do Indian parents pick young girls - older boys as a potential couple for marriage?

Some do it without thinking as it's a good old tradition. Logical as far as reproduction is concerned as it gives more time to the couple to plan their next generation. Indian system is mostly patriarchal, which wants girl to follow boy's lead. If they are of same age then girl might question. People want to avoid social questions arising when this is not true i.e. a girl is of higher or same age. Parents think older (<=5 years) the man more stable his income will be (keeping in mind that he is not too old) Apart from this there are actually a lot of potential perceived personal as well as social compatibility preferences which are taken in account if the decision is left to parents :).

Perhaps, we have been conditioned to think this way from centuries and centuries that girls should be younger and boys elder to them. Secondly, in our society, the mindset is such that girls mature early and boys late, so if one already marries a mature girl, she is going to get more mature and look old after bearing the child and as the years progress.