Term limits on the presidency have nothing to do with age. You could be 35, serve two terms, and you’d still be banned from being president again … at age 43.
It’s not. Nobody is saying old people are subhuman scum who shouldn’t be treated kindly and with respect. The argument is that after a certain point you may not have a lived experience that is relevant anymore for being an effective political leader. The same way we wouldn’t put two 80 year olds in the cockpit of a commercial aircraft… the risk there is obvious and clear.