Well, it depends on what you mean by "general." There were a series of command posts, starting with military tribunes. In the Late Republic, these guys seldom did any commanding, but they were generally in their 20s, at the beginning of their careers. In the 1st century BCE, the functions of tribunes were regularly replaced/supplemented by appointed legati. They could be anybody, but tended to be a bit more experienced.

Praetors held imperium and could therefore command legions. The minimum age for a praetor was around 40 during the Republic, so a typical general with imperium pro praetore would be in his mid-40s, theoretically.

Most commanders were, of course, consuls or proconsuls. The minimum age to be elected consul in the Republic was around 42. That means that generals who were also consuls would be about 42; those serving afterwards, with imperium proconsulare, would be in their mid-40s.

Now, Sulla via the Senate gave Pompeius imperium pro praetore when he was only 23, then sent the kid against his enemy Carbo in Sicily and then Ahenobarbus in Africa. Pompey was the general in charge in that campaign and when he returned victorious to Italia, he politely declined Sulla's request to disband his troops. Sulla relented and the kid was granted a triumph in the capital at the ripe old age of 24.

In other words: these are general guidelines, especially after Marius.