1. let vote only people who contribute (pay some minimum income tax) at the time of elections - no unemployed, no students, no retired people - regardless the age, so if you pay your taxes at 14 you are eligible to vote at 14, if you pay your taxes at 88 you have right to decide what will be done with your money at 88, actually I would go even so far that EVERYONE regardless of citizenship should be allowed to vote if they are paying taxes, either they are good paying taxes, so they should be good also for voting
2. other approach would not be tied to income tax, let vote absolutely everyone regardless of age, so even children get vote, so family of 4 with 2 kids will get 4 votes, so they can vote for future of their children and not that retired people are deciding (usually selfishly) future of the kids
I am aware of reality that 1st option would cause huge uproar and second option is more realistic with some countries moving that way lowering voting age to 16 for now.
At the same time I would be glad if we would get rid off Pay-as-you-go pension plan and replace it with own savings account (IRA), but it seems no country in the world had balls to switch, even Chile had to backpedal a bit, but they got the furthest. It's the only solution for demographic changes.