So, at what age are women having their first child? The median age of mothers giving birth to their first child is roughly 28 years, two years younger than the average across all mothers.It is also important to note that this median age is based on children of the current relationship only.he median age for both childbirth and birth of first child has remained steady since 2001. About one-half of births are registered as first births. The median age of women giving birth to their second child is roughly 32 years.