The idea that the right to vote is tied to your tax contributions is very flawed.
Paying taxes without the right to vote is absolutely ridiculous (so either link the right to vote to the age you're allowed to work, or tax exempt any work done under the voting age), but the inverse is ridiculous as well. People, above the voting age, that don't pay any taxes for whatever reason, should not have their voting rights stripped. This reasoning gets dangerously close to a plural voting system, where you get multiple votes if you're rich enough.
If you want to disenfranchise retired people, use some other reasoning (like decline in cognitive abilities), not because they are no longer actively paying taxes.
Note that I am not in favour of disenfranchising anyone. Keep the lower limit for voting age, or even reduce it, and no upper limit. Also make voting as accessible as possible.