The compulsary age for staying in education is already going to be raised to 18, forcing some young people to complete courses they may not be interested in.
Forcing young people into the military after this would mean one of the following-
1. A set of young people who have been forced to spend 4 years of their lives doing something they don't want to do, leaving them with very little trust in 'the system'
2. A large amount of people not entering university until they are 20/21, these people would still rely on financial help from their parents until they were at least 23/24 making the 'apron strings' problem worse.