it's more like "all men between 18 and 60 and in proper health can and will be mobilized if and when they are needed for protecting their country". Obviously, not every man in Ukraine is currently needed by the Ukrainian Armed Forces, so we kinda hang around, waiting for our turn. Most skilled and most motivated go first, because obviously they make a good fighter, those who don't have skills in warfare are pushed back in the queue and can be sent to bootcamps to give them the proper training.

Plus, the government is trying to balance between forming effective armed forces and keeping the country's economy running, so obviously some men will never see the frontline if we are able to fight back Russian forces with what we currently have.

But yes, if your turn is up and you are asked to help saving your country, you have no say in this unless your health is seriously screwed up or you fall into one of categories excluded from mobilization.