The Bible does not tell us how old Mary was when she was told she would give birth to Jesus. But we do know she was very young because in those days it was the custom for girls to marry very young. Most Biblical scholars believe she was around 15 years of age.
The reason for this custom was because by age 40 they were considered old and 40-45 was the age span in those days.
LadyE a good point you added; about the life span during this time.