The Eberron Campaign Setting on Random Starting Ages (27) lists a warforged's random starting age as 0 then +1d12 if its first class level is in a simple class like barbarian, +1d6 years if its first class level is in a moderate class like bard, and +1d4 years if its first class level is in a complex class like wizard.
This is the exact opposite of how random starting ages are determined for other races: they enter play older the more complicated their first class. (This seeming weirdness goes unmentioned by the Eberron Campaign Setting errata.)
Was an official explanation ever offered for warforged starting ages? That is, did a designer ever say, for example, that because of, like, a warforged's magical-mechanical brain, a warforged takes longer to learn how to act on instinct than to learn how to perform complicated intellectual tasks?
Best Answer
It's a subtle nod to the fluff
From page 22 of the Eberron Campaign Setting:
Aarren d'Cannith created the first warforged 33 years before the present. They were mass-produced and sold to whoever could afford them, as squads of basic infantry. It took a long time for people to realize the potential of warforged, and order batches of them who're trained in more complex roles. Warforged receive training in their creation based on their intended role, and this is reflected in their starting ages.
The younger the warforged is, the more likely they were placed in a role other than direct, frontline combat, and most warforged 2 years of age or younger weren't created as soldiers at all—they were made in secret against the terms of the Treaty of Thronehold. As such, they tend to be free to choose whatever life path they want (even if their creator wanted them as soldiers, it would be difficult to do it without a hammer coming down on them).
As for how warforged manage to learn complex classes at a young age: according to this article by Eberron's creator, "The Warforged, Part Two," newborn warforged are utter sponges for information.
It isn't stated if the oldest warforged had the aptitude for magecraft and similar things; over the course of the past 33 years, warforged technology continued to advance, with limited-run specialized squads being ordered occasionally, new models (like the warforged scouts and chargers) being designed, and so on. At the very least, for the most recent warforged, when assigned to learn a more complex task, they simply can, naturally and intuitively.
It's possible that the oldest warforged had similar aptitude, but didn't become mages because of the jobs they were assigned to learn, but I haven't found anything solid that states it either way.