I'm so glad he was aged up. It's a very profound, bittersweet and badass moment. A true sacrifice unless Bonney undoes it lol.
But since Hiyori had to live out those 20 years, I think he'll stay aged up and let his mind mature and catch up ASAP and he's technically older then her so its better this way too.
It's a similar storyline to Ocarina of Time. Boy sacrifices youth to become a man and necessary hero.