Grandson is 20 years old. How old is grandpa?

According to the Marriage Law, the age of marriage is not less than 20 for women and 22 for men. The best childbearing age for women is 25-26 years old, but not over 30 years old. According to best age for marriage, if grandpa gave birth to a son around the age of 25, the son will also be born around the age of 25, and grandpa will have a grandson around the age of 50. But this is not absolute, generally depends on the time when people get married and have children.