We dont get clear info about their ages, but the conscription age is typically at the age of 17-18 according to the needs of alfheimr. From the info in chapter1, Britt and Borge just turned 18. In the faction page of the main menu you can see how many years passed since each captain was conscripted.
All the children are being sent to foster families as babies. Jorgen grew up in the same foster family as Britt. They are not related by blood.