484 - Justin is born.
494 - 10-year-old Justin squires for nights in Arthur's rebellion against the Saxxons.
500 - Camelot is built.
502 - 18-year-old Justin is knighted by King Arthur.
504 - 20-year-old Justin helps Merlin defeat Morgaine Le Fay, and is gifted with his magical armament.
511 - 27-year-old Justin is cursed by Morgaine Le Fay when he discovers her treachery, sealed in suspended animation within a tree.
37 years ago - 27-year-old Justin is saved from his curse by the All-Star Squadron, and joins them.
24 years ago - 40-year-old Justin is one of the final members of the All-Star Squadron when it is finally shut down.
22 years ago - 42-year-old Justin stops Morgaine Le Fay from freeing Mordred from the paths beyond, and chooses to adventure within and quest for Avalon.
There is a glorious silliness to the Shining Knight, but it's exactly the sort of idea that sometimes makes comics so grand. Simply by making him a time displaced Arthurian Knight, he single-handedly has introduced the very existence of Arthurian legend into the DC lore, and we get to milk that to great effect all over the timeline. What's more, by including the idea that Percival Sheldrake was his squire (we thought we had invented the idea, but it turns out that it was already cannon), we get to use him as a creative impetus for a whole group of legacy characters that are wildly fun but would feel pretty pointlessly redundant without this character to tie them together.
In the comics, Sir Justin is one of many characters that were active during WW2, who didn't quite ever have their continued career in modern times explained thanks to constant multiple world fudgery. Like many of the characters we were looking to maintain a tie with modern day legacies, we've moved him into the All-Star Squadron, and he fits quite perfectly into that nitch. In order to advance the story for Percival and his son, we needed Justin to have left the world of mortals, but rather than having him die, it seemed so much more epic for him to have "departed the mortal realm to quest for Avalon". Just writing that felt cool.
All that said, there is something just so fun about a character with an enchanted flying horse and a magic sword. The idea is so pure that we identify with it immediately, and as out of place as he might look alongside the Batmans and Amanda Wallers of the world, He's a fantastic inclusion in a world of Green Lanterns and Aquamans.
We do really need to mention the other version of Shining Knight, Ystin. Created in the Seven Soldiers of Victory miniseries event by Grant Morrison, this version of the character existed almost completely ancillary to Sir Justin, with an entirely different backstory rooted in a proto-camalot (?) loaded with farie magic. Ystin eventually was revealed to be Ystina, a woman disguising her gender to join Arthur's Knights, which is downright baller.
Ultimately, Sir Justin fits into the larger DC continuity much better than Ystin, and there's really no way to have both versions of the character.... but that doesn't mean that she isn't an awesome, awesome character.