I know modern day mothers in the developed world are getting older and older (often 30+) whereas my grandparents generation (born in 1932) were having children in their early to mid 20s. How has it changed over time? From how young to how old?
A roman girl would probably marry around 13-14, 12 at the earliest, and her husband would probably be a 16 year old (if he was marrying for the first time). Babies would follow soon after. There are exception to this, Augustus' second wife and the mother of his only natural child Julia was both older than him and gave birth at the age of 29, quite old for a roman woman.
Even back in the 18th century it was in their early 20s. Doesn't really change until dependable birth control becomes available.