The minimum age Of a presidential candidates must be:

Age of candidacy is the minimum age at which a person can legally qualify to hold certain elected government offices. In many cases, it also determines the age at which a person may be eligible to stand for an election or be granted ballot access.

The first known example of a law enforcing age of candidacy was the Lex Villia Annalis, a Roman law enacted in 180 BCE which set the minimum ages for senatorial magistrates.

There is no fixe age. For example in United States the minimum age is 35 but in Estonia every citizen who is 18 can be elected as president