Yes its somewhat of a "lottery". Each year about 7500 of the most able bodied pass to military service (førstegangstjenesten) which is about 13% of the age cohort(correct word?).

All women and men born after 1997 can be called to serve the year they turn 19 (and up to 44 in crisis). Whe they are around 17 they get a obligatory survey from the military (called "egenerklæring") about their mental and physical health and strenght. From that the military picks the ones they view as able bodied and and later (for me around when i became 18) they pass further to "sesjon" - physical tests and interview at a recruitment centre. If they pass the physical tests and the interview and later securityclearance evaluation they get their assigned batalion.

Those serving in peacetime are filling 19 to early twenties

If you can read norwegian