Why the girl should always be younger to the boy, for marriage?

The Ministry of Women & Child Development has constituted a taskforce to consider the correlation of age of marriage and motherhood with implications on the health of mothers and infants, impact of the age of marriage on infant mortality rate, maternal mortality rate, total fertility rate, sex ratio at birth and child sex ratio.
The taskforce headed by former Samata Party President Jaya Jaitly and 10 members which is due to complete the task by July 31, 2020.
There appears to be no social angle to the study. Why can’t the ages of both, bride and groom, be same? Why the girl always has to be younger to the male? Why the age factor is linked only to child bearing or mortality???
Is it because the girl should always be respectful and eager to serve the husband!