"Get gud" :)
No but seriously, I have no idea. I'm in my 40s and have potato reflexes. I've got friends that range from Unranked to mid Plat. When I play with them they all make mistakes, but what I notice is the higher ranked players:
A) make fewer mistakes
B) make better calls, like when to team fight, take Dragon, towers, when to chase, etc.
C) are far superior at jungling (those who jungle anyway)
D) play a narrow selection of champions
E) know when they hit their "power spikes"
F) can adjust their builds easily
G) ward better than me, in better places and better times
H) tend to carry games
I) play mostly ranked or draft normals (some refuse to play ARAM, saying it lowers your skill)
J) will dodge a bad team comp
K) use Skype whenever possible
L) don't feed
M) don't linger in base more than a couple of seconds
N) get FAR better creep score than me
O) are younger than me (mostly)
P) make the most of crowd control
Q) have superior positioning
R) play every day, multiple games
S) focus carry and squishy targets
T) focus healers
U) play high mechanic champs
V) don't rage often
W) roam often
X) upgrade their trinkets
Y) use the terrain to their advantage (bushes, walls, traps)
Z) have good synergy with their team
There's the A to Z, in my opinion. How do you learn it all? No idea.