It's a subtle nod to the fluff
From page 22 of the Eberron Campaign Setting:
Older warforged tend to be fighters or barbarians. The more recently created warforged, especially those less than five years old, are more inclined to try different class options.
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.
The warforged is born a blank slate, but it possesses a voracious appetite for knowledge. Just as the mind of a human child is more flexible than that of an adult, in its first few months a warforged possesses an astonishing ability to learn and adapt. A warforged may begin with no knowledge of the battlefield. But many Cannith instructors claim that the knowledge seems to be hidden beneath the surface, just waiting to be unlocked -- that within a few months of training, a warforged soldier may outstrip the skills of his teacher. The warforged must be shown the path to walk, but after taking its first steps, a warforged often picks up speed, learning skills in mere months that a human might take years to master.
After the first few months, however, the warforged's thought patterns crystallize, fixing on the path it has chosen. At this point, it is essentially an adult: It possesses the skills it needs to perform its chosen task, and while it can still learn new skills, this takes just as much time as it would for a human or an elf.
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.