Broke: Gilan is 12 in TEY, which makes him closer in age to Halt than to Will.
Bespoke: Gilan is at most 8, it's just that the primary source of information on the topic (aka sir David), being an average dad, has only the vaguest idea about his child's age.
Also consider: David, a single father during wartime, could be tempted to bend the truth a little about his son's age if it would allow Gilan to stay with him and the army--rather than leave his child behind and alone in a war-torn fief subjected to constant raiding by Morgarath's troops and Wargals.
Either way would definitely make those vast age discrepancies across the books a little less egregious/noticeable.