1) He was never technically an Avenger. Clint only gave the "If you walk out, you're an Avenger" speech to Wanda and no else even recognized her as an Avenger until the new Avengers.
2) He is only 24 and he got his breakthough role when he was 19. Use your imagination.
Training. He comes from a long line of Samuel Jacksons, each more badass than the last. L is not an initial but a roman numeral designating his place in their family.
Aye, that it does. See, because of the similar numbers, some people associate them with the American presidency, but they fail to take into account that a Jackson lives longer than a president serves. Their line actually goes back almost four thousand years, to when Stonehenge was completed. It's not commonly known, but the structure was actually a device meant to harness the badass of several Celt and viking warriors into an uberwarrior who could defend the world from an incoming darkness that only the wise and mighty druids saw approaching. They instilled that power into a tree carved into the shape of a man, and its well oiled ashen surface is what gives the Jacksons their magnificent complexion that never seems to break for anything but the coolest of scars.