Jin after flying away from 21, found a mountain where he wouldn't be disturbed. Once he reached he immediately used the dragon balls to summon Shenron.... who was massive.... like really big, the anime didn't do him justice as he made the mountain look small.
Shenron: State your wish
Jin: I would like eternal youth please!
Shenron: It shall be done
Shenron's eye's glowed for a moment, before he disappeared and turned back into the balls, which turned to stone, however before they could disappear, Jin quickly caught all of them and placed them in his bag, and flew back home, where he would teach, train and attend school
10 Years Later
By the time Jin was 15 years old he was 5ft 10, his pitch black hair was just like Grimmjow's from bleach, Thanks his looks, and charm, and brains, he was considered to be irresistible in the eyes of his female classmates and... cough cough... their mothers cough cough...
Jin spent every free moment he had training in the gravity chamber, and unlike Goku and Vegeta, he was able to dial up the gravity much quicker than them, and progressed way faster. Jin had managed to reach up to Super Saiyan 3, and completely mastered it in ways that Goku couldn't. The way he did this was by making sure there was no excess ki leaking. Although his body did change when he entered super Saiyan 3. Instead of becoming stupidly big, he reached a height of 6ft 2 and his muscles grew slightly bigger than they already were.... and he also kept his eyebrows
Jin however wasn't satisfied with this as he wanted to be as strong as Super Saiyan god in his base form. He wanted to be stronger than Jiren was during the tournament. Jin would keep training the Briefs, and help them get stronger. Right now they could all fly, and could fire ki beams from their hands. Jin was sure that they were the strongest humans on the planet right now. The reason why they were so strong so quickly, was due to Jin being a master teacher, he understood the best ways to train them individually, and made sure that they could understand everything, and managed to help them improve drastically
Bulma wasn't anything like how she was in the anime. She wasn't interested in other boys.... probably since she was in love with Jin. She also had a positive look towards learning martial arts, and progressed the most out of all of them, which could be due to wanting to impress Jin. On another note Jin started to go to school with Bulma, and they were both already recognised as geniuses. Bulma because she was a natural whiz in all her subjects, which didn't surprise him at all. However Bulma on the other hand was surprised when she saw Jin pass all of his subjects with flying colours
Jin became very well versed in all of his subjects, even surpassing his teachers in knowledge, which resulted in them calling Mr and Mrs. Briefs, who were classified as his guardians and were told of his high IQ. However before they could say anything, Jin refused, as he said that he didn't want to leave Bulma, and would like to stay in class with her. Bulma who was feeling down, was really surprised at first, which then turned into happiness
Although the teachers kept trying to push Jin into graduating, so that they could seem impressive in the eyes of the school board, Jin was very adamant on staying behind. Jin would spend his time with Bulma and Dr Briefs, and help them with new inventions, it was actually Jin who managed to fix Dr Briefs problem on how to store a home in a capsule and had actually developed a battle power scouter, which he used on them
Bulma - 475 BP
Tights - 385BP
Panchy - 403 BP
Dr Briefs - 401 BP
Jin was very proud of his work, and how far they had come. Their bodies were way stronger, and Bulma had a much better body, with curves in all the right places, and a slightly bigger bust. Another thing that Jin had done was write books... yes books. He didn't want to be like Goku and Vegeta who lives on his wifes families money, he wanted to earn cash for himself, even though he could create money out of thin air, he wanted to keep busy when he wasn't training
Jin was very famous for his books on Harry Potter, Lord of the Rings, Game of Thrones, Star Wars, and Chronicles of a Shinobi. He was very famous worldwide, and would often help Tights on her books, and managed to introduce her to some publishers who were more than happy to take in a friend of the legendary Jin's, who'd sold millions of books worldwide
About 9 years ago Jin went back to Gohan to return the dragon ball, where he met Goku, who was 2 years old at the time. Goku immediately took a liking to Jin, who would play with him and dote on him like a little brother. As much as Jin tried, he couldn't get rid of Goku's carefree nature, however he certain that he'd be able to teach him basic maths and literacy.... no matter how hard it was... sigh
Jin had met 21 again a few times during his days in school. This was because the Red Ribbon Army had heard of a young genius and wanted to take him in. At first 21 wasn't really bothered about it, however, when Dr Gero gave her a picture of Jin she immediately left to go find him
Gero was surprised. For the past few years when he tried to spend time with her she'd just brush him off, even though she used to always be around him, she wasn't anymore, it was as if she didn't like him. Although Gero didn't have any romantic thoughts about her, she was a very useful researcher, and he'd planned on doing some experiments on her, so he'd need to keep her by his side, but now, she'd just keep her distance, and was always gazing at the sky
When she'd arrived at school and found Jin, she started to blush immediately as he was a 15 year old boy with a killer body, and wanted to mount him as soon as she saw him NO!... bad 21 no..... Although Jin refused, she didn't mind, this way she could spend time with him trying to 'convince' him to join them