I believe that he came to Earth/into her care when he was still a child. It’s just that time moves differently on Earth and the world where he was born. So while he had only stayed with Jay on Earth for a few months, it had actually been years in his world. While he was living on Earth, his physical body in his original world continued to age (since it was his soul that had come to Earth, not his body) and when he returned to his original world, his body was that of a 17 year olds.
Because time goes faster in his world than on Earth, by the time Jay was transported to his world, 10 years had already passed. So physically and emotionally he’s that of a 27 years old. It’s just that time goes slower on Earth, so it had only been 4 months for Jay, while in his world, 10 years had passed.
Well ya'll forgetting they said it's been ten years so louie was 17 when he left so it doesn't seem that bad and also I'd you didn't know there are fricking sugar babies and daddies so.......