If someone has a clear career path at a young age and can benefit from training and experience, then no, don't force them into national service. By school leaving age, it's obvious if someone has the talent and fitness to be a professional sports player. Sport should be an acceptable exception to national service. Opposition to military is another, you can't force people to back down against their core moral principles. That's just a shit thing for a civilised state to do. It's not civilised. So a humanitarian option, such as search and rescue training, advanced first aid, things like that, or even a community option like litter picking and park maintenance for the stupid and stubborn, so long as you're not forcing people into the army against their will and principle, or holding talented youngsters back from realistic careers, then ok. Just be civilised and modern about it.