Maximum age for president amendment: 70 when term starts.

There is already a minimum age for president: 35.

Article II, Section 1, Clause 5:

No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States.

Why not a maximum age also?

70 when the elected term starts on January 20.

As a practical matter, voters want to consider their candidate serving two terms. So to be eligible for a second term, the candidate would need to be younger than 66 when the first term starts.

That leaves plenty of practical room. Who really wants to elect a president older than 66? What organization would select a chief executive 66 or older?

People can start collecting Social Security payments at age 62. Why elect a president who should be retired?