The story begins in December when Meg was 16, Jo 15, Beth around 13, and Amy 12. Mr. March was wounded in November of the following year. It was around this time that He Hummel's baby died of scarlet fever, and Beth also had a close call with the disease.
Mr. March came home early the following year (Jo would have been close to 17).
Meg was married at 21 (the three year gap between Book 1 and Book 2). Her twins, Daisy and Demi, were born about a year later. Beth died when the twins were around 2, making her about 21.