how to predict date of marriage from birth date of a person?

I don't think that can be done. You could guess a ball park figure by going up maybe twenty years for women and twenty five years for men but it would strictly be a guess as to the exact date. I guess a fortune teller could tell you.

Marriage is a human/religious/political invention, it isn't a "natural" part of the universe like natural events and personalities. I don't think the stars hold any information on marriage.

Especially because marriage means something else for everyone.... even if you could determine when someone would find a stable relationship, you wouldn't be able to tell if they got married. And even if people get married, it doesn't promise that it's lasting so it's just not astrological data.

This cannot be done with any accuracy. If there were some magic formula, that would be that all people born on October 1 1979 at 7:00 pm would get married on the same day. It just doesn't work like that.