Yes, 25 was old enough in legal terms but for running a 60,000,000 people, 10,000,000 km2 empire ? I think not .
I realy can't find any "good" emperor who came to that position young . Augustus is a unique case and was 36 in 27 . Alexander Severus was 14 in 222 but was he "good" emperor ? same for Gordianus III who was 13 in 238 .
Constantinus, a "good" emperor, was 32 in 306 (not so young) but took the whole empire in 324 when he was 50 . And on .