Who decides how old is "too old?" And what happens when the people approaching that age decide that young people (18-30) are the real problem and remove rights from them in retaliation?
This is the sort of thing that people shout in anger into the mirror at night, but any actual plans fall apart immediately.
Never give your political allies a power (in this case, removal of voting rights) you wouldn't also give your political opponents. Your "side" won't always be in charge, and what sounded good to yourself may be terrifying when wielded against you.
↑↑↑
The median age of people 18+ in the US (total potential voters), is 47. The 80+ demographic is really small. If those folks are voting in ways that are detrimental to others, the solution is to increase voting among other groups.
Make election day a mandatory holiday for anyone outside first responders, and guarantee those folks a half day. Expand mail-in voting. Expand early voting.
There are dozens of solutions that don't include telling people they don't get all the rights of full citizens.