Finland has mandatory service for all males which has to be completed before the 28th birthday, but it’s usually done at the age of 18 or 19. More than 80% actually do armed service. The others get a free pass due to a medical condition or are conscientious objectors. The conscientious objectors have to take a short course and do about a year of work for something like a hospital, library or school. Those that refuse this go to jail.
A few years ago it was changed so that most ordinary soldiers serve for 6 months. Some with special duties (drivers, medics, MPs, etc) do 9 or 12 months. Those who get leadership training are also in service for 12 months.
There are also refreshers after the actual service. They should be about 5 years apart but it’s quite easy to weasel out of them, some are even voluntary.