You do remember for those arcs you mentions Ace was only sitting on his ass or seen as a child
As JC already mentioned, this is exaggerated. Through those arcs there was a great amount of time centered around his life, his relationships, his development as a character, and his effect on the story. I don't see how his age affects anything. Sabo only had a role in Luffy's/Ace's flashback and he only appears periodically in Dressrosa.