"when he would be old enough to stand for a praetorship "

The age requirements of the various magistracies on the cursus honorum (political career ladder) were:

Quaestor – at least 30 years old

Aedile – at least 36 years old

Praetor – at least 39 years old

Consul – 40 years old for a patrician, 42 for a plebeian