I'd argue earliest age at which you can be drafted minus maximum term length (ie. 5 years in the UK) sounds good as a general rule. Otherwise people can get drafted by a government they didn't have a say in electing.
I'd also argue that there should be no lower age limit for voting for people with taxable income. No taxation without representation.
> Otherwise people can get drafted by a government they didn't have a say in electing.
This kind of thinking is not rooted in reality. When you were born, you were forced to accept the conditions you were born into. The same is true with laws. I understand that going to war is something else than going to school, but that's life.
> No taxation without representation.
"Representation" in the original slogans context does not really apply here (since it was about voting rights for a taxed population as a whole).
But for the idea of "anyone who has to pay taxes, must also have a say!", I can only say that it comes right back to the previous point: You could just as well argue the individual income tax rate must be zero, until a person had the opportunity to vote at least once. The world doesn't work this way.
I'd also argue that there should be no lower age limit for voting for people with taxable income. No taxation without representation.