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 soldiers. It took a long time for special variants to be made and for warforged to begin to be given more complex roles in the armies they were purchased by. 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).