Cos they're less 'moldable' and less likely to put up with BS, more likely to be picky (or less likely to 'yes' to any guy their parents stick in front of them depending on which way you look at it), their standards are likely to be higher, less likely to be emotionally blackmailed or manipulated as they tend to be more educated, more financially independent and less naive about things.. The thing about not being able to have kids only applies to women who are quite a bit older and obviously the vast majority of desis are married by then.. even if they choose to wait..
I actually had an aunty try to convince my parents to not let me pursue medicine with all these reasons..."jaldi se shadi kar dein because..."
I wanted to kill her. It really is all about the fact that an older woman TENDS TO BE less dependent on her spouse and therefore has the ability to look at rishtas critically and isn't "moldable".
I think it has more to do with the religious aspect of it as well
Religion advises all muslims to get married ASAP when they come of age. The earlier the better. There are quite a few benefits to this, not the least of which is channeling sexual energy which comes into play when a person matures into a young adult.
That aside, there are biological benefits to getting married earlier, having kids earlier. The later it gets, the more likely that children will follow at a late age as well and that is harmful for the mother's health. Twenties are a safe zone to have children in. Beyond 35 it is harder for the woman to concieve (as opposed to in her twenties) and is more dangerous for her as well.
That aside, younger parents can still expect to see their kids marry and see their grandkids in their own lifetime. A couple getting married when the girl is say 33, would likely have kids when girl is ~35 yrs. If they have more kids, say three, then it is safe to presume the youngest will be when girl is 38 minimum. When that child gets married (lets say at 20 years), the parents would be quite old.
:P Random math on my part. Im not even considering the dad's age in all this.
Another issue with late marriages I have noticed in our culture is that the proposal pool diminishes rapidly. So that at the end of the day when you begin looking for a rishta, you get slim pickings. Most boy's families would want young girls. Boys available would either be already divorced, major players or some other random reason as to why they are unmarried at a late age.
^^Islam only advises early marriage if the guy can support his wife and himself, provide housing etc. Our culture loves to emphasise early marriage being encouraged but conveniently forgets it comes with conditions..
I agree with Demesne that a lot of it has to do with the biological clock. As mentioned the older you get the harder it is to have kids. Especially for women, guys can technically have kids till they're like 70 which is why a 35 year old guy can still look for a 21 year old. Even looks fade with age right so people are a lot more attractive in general when they're 25 vs 35.