The alignment of the two ages is actually pretty recent, at least in the US. Traditionally, military conscription age was 18, while voting was 21. It was moved (in 1971) due to agitation from people who argued that it was unethical to conscript people to die for their country, but not trust the same people to vote for their leaders. But that's merely an argument for why voting age should be <= conscription age, not necessarily an argument that it should be ==.