The Serpent Society is a well funded terrorist group of reptile themed supervillans.
They're a JSA enemy.
The Serpent Society is a Marvel comics entity who aren't really terrorists but do use reptile themed villains
You're getting them confused with the Kobra cult, who are a snake themed terrorist group who also make use of non-snake supervillains and are DC characters
The QM has told us that learning Magic at Lex's age is usually impossible.
I'd like to tone down the statement from "usually impossible" to something a little less harsh (for example Sam learned magic and he's of a similar age) but it's exceptionally difficult for Lex Luthor because of who he is and the way he grew up and views the world.
Lex specifically has a harder time with magic than most people would because of his mindset.
Edit: that's not to say most adults would have it easier but someone who was raised from birth in a culture that believed that magic was real and could be tapped into in a set way because of some reason would have an easier time than those raised in a skeptical society. People like Bruce Wayne, Victor Fries, Selina Kyle, Barry Allen, Sinestro, Mr. Frost, Veronica Cale and Lex Luthor would all have a harder time learning magic than lunatics like Jervis Twitch, Murmur and Humpty Dumpty and then all of them would have a harder time learning magic than a true believer in magic and then a true believer in magic would still likely have a harder time learning magic then a sufficiently openinded child.
TLDR: multiple factors are at play to make it harder for Lex to learn magic.
But how mirror master have magic got it out random, and he is adult. And Not us
Sam got it because he fundamentally didn't understand how mirrors work. Sam fundamentally misunderstands how reality works and uses magic to bend reality to match his misunderstanding.
Lex fundamentally cannot do the same thing because he knows how the world works and because any attempt he'd have in using magic, causes him to look for underlying principles which aren't there, thus causing him to view magic as an "impossibility made manifest" which in turn makes it impossible for him
Edit: the reason why most magic users need to learn magic as children is that their mindset is more malleable. They have a lot more latitude to reject mental schemas of reality and compensate with magic to make their own than adults do. Lex inherently needs to combat his own way of thinking about things in order to use magic and that's very difficult to do.