The only information available on it, that I'm aware of, is in the 2nd Edition AD&D Complete Book of Humanoids. Minotaurs are one of the races described therein, and in the appendices there's a listing for their ages. Starting age is 12 years + 3d6 years, for what I assume would be a young adult. Their maximum age is 150 years + 1d100 years. Minotaurs are middle-aged at 75, old at 100, and venerable at 150.
In general though, you won't find age entries for most monsters in D&D, since those only tend to show up in the descriptions of standard playable races. Minotaurs only got an age listed in 2E because they were in a player's supplement made to add monstrous humanoid races to the players' repertoire.
So just guess or houserule it for most cases. Dragons are one of the few monsters that have ages listed in 3rd edition, because it's important to their abilities, but it's not particularly important most of the time for other monsters.