Characters are their canon age when they show up then age one year per expansion, adjusting as needed to make relationships make sense. The only weirdness this has caused so far is the twins going from three years younger than Wuk Lamat to two years older.
#wolqotd (kinda) The Encyclopaedia Eorzea books provide ages for NPCs: how do you "treat" this piece of info within your own wol's lore/timeline? Are they canon? If so at which point in time? Or do you adjust them to your personal timeline? Or perhaps it's all the "1 year time bubble"?