In Ancient Rome, the two highest political offices in the Cursus Honorum were Praetor and Consul. The minimum age for becoming a Praetor was 39, and for Consul 42. My source reads as follows:
To have held each office at the youngest possible age (suo anno, "in his year") was considered a great political success, since to miss out on a praetorship at 39 meant that one could not become consul at 42.
If Praetors served for one year, why were they not able to do so at age 40 or 41, and then serve as Consul at 42?
Uh, I guess technically that's true, but just because of the way years line up. A plebeian could stand for the consulship at 42 (39 if he were a patrician), and a year was supposed to pass between the expiration of a previous magistracy and the election to a new one. Cicero Fam. 10.25 seems to suggest that the reasoning is to allow a full year of campaigning between the expiration of office and the elections. Therefore, effectively, two years were supposed to pass between the election to one magistracy and the election to the next. If a (plebeian) candidate had held the praetorship at 40, having stood for it at 39, then effectively he was not pragmatically eligible to stand at the elections until 42, sitting as consul at 43.
There was no reason that a candidate could not stand for election past the earliest possible age, you're misreading it. If a (plebeian) candidate had not stood for the praetorship at 39 and instead had done so at, say, 40 then he would be sitting as praetor at 41. He would therefore be eligible to stand for the consulship at 43, and to sit as consul if elected at 44. And so on.