Depends. Wizards yes, rogues and fighters (or sorcerers) not really.
In 3E different classes had different minumum ages for PCs, indicating their training
Not fighter or rogues? A rogue supposedly has skills in picpocketing and lockpicking (not to talk about stealth and backstab) that requires experience to master, a lot of it. Fighters can use all weapons, swords, maces, bow, shortbows, spears and so on how can do that without training? Even when wars explode and states force enlist to increase their army the new recruits go through (even if fast) training.
