Not a particularly convincing argument for several reasons
First functionally no one is saying old people have no right to run. Some people are saying they shouldn't. I can absolutely take age into account in how I vote.
Then there's that if we took this to the logical other end, do you give twelve year oldsthe right to vote, to drive, to drink, own guns, work in mines, or consent to sex.
We know that on average people change on a bell curve. For a while we as a society hold that while maybe an exceptional 12 year old could drive safely, it's a tiny percentage and it's fine for that percentage to still not drive. Likewise as we get older, statistically we all slow down. Older drivers in my state may be required to have restrictions on their ability to drive. In Virginia older drivers are required to retest more frequently and have different requirements because statistics say that they have a greater risk of being a danger to themselves and others on the road.
Would you vote for a 12 year old for Congress or president? If not, isn't that ageism?