^^^^^^^^^^^^^
THIS!!!
Not to sound mean, but I had to fire a guy for doing the exact same thing. He was older than you and a pretty good tech, but when the customer has no front brakes at the bottom of a hill on a 4 lane highway, less than five minutes from your shop, you gotta let him go...
Tommy
I am not sure of your legal obligations and understand that there are risks, but becareful of your reactions and firing someone because of their supposed Learning disabilities, that could have legal implications of their own. We are all human, and while a seasoned mechanic may be at less of a risk for making a mistake, he is just as capable of forgetting to do something as the next guy.
I have some experience with this. I have to make a concerted effort to go over everything that I have tightened and before I progress to the "next step" something that would cover over a place that I have previously been, I ensure that I have everything tight. Absolutely do NOT HAND TIGHTEN ANYTHING and then go back over it all with your ratchet, or air tool - start everything by hand, but make sure you leave it only one or two threads in, so you can see that it is loose. Another big one is to not use air tools, or electric tools, this forces you to slow down and ensure everything is tight, and lets you "feel" the tightness...
As a beginner it is CRUCIAL you slow yourself down, until you can do that process in your sleep, three or for times over, then you can start to speed up and try to make time on things. It takes time, and no one on here can say they have NEVER made a mistake, it's who we are, and unfortunately this is a part of learning and becoming a better mechanic. You will be twice or even three times less likely next time to not do the same thing.
Keep your head up.