You mean like how most places don’t let you vote before you turn 18 because it is accepted that children have not developed the cognitive ability to make sound decisions in regards to electing officials?

This is a strawman argument. OP was talking about an age limit for elected officials, whereas you are now talking about age restrictions on the voters. Yes, we are both talking about cognitive decline in decision making; however there is a substantial difference between putting an age limit on those who can be in power vs. putting an age limit on those who can decide who is in power.