That's already how it works. You never have to register to vote; you can spend you're entire life opting out of the electoral system.

But when you register to vote you've accepted your civic duty to cast your vote everytime. So 16-year-olds would just have the option of starting earlier.

I've gone back and forth on this. I definitely think they should have the option to vote, but I'm not sure whether it should be compulsory or not.

The main problem I have with making it purely optional is that if you didn't, that could create a sort of smaller-scale American-style campaign where the goal isn't to make the best case, but to make your campaign about encouraging your supporters to vote and discouraging those who disagree from voting. Obviously they'd only be doing that for 16 & 17 year-olds, but that could potentially still be a problem. I don't want politicians to have any more incentive than they already do to run shitty negative inflammatory campaigns.

Another potential problem is around voter education. People are really dumb. So many people already have problems understanding how our voting system works. We want to send out a message that's as simple as possible. "Everyone has to vote" is a much simpler message than "16 & 17 year-olds can vote if they want, people aged 18+ must vote."