Actually King's Maker itself is an all ages version (shounen ai like what everybody said)
This MATURE vers is like a "bonus" for fans, which is a full version (including SMUT scenes) of Season 2 that was released few months after the all ages vers finished.
No mature vers for S1. Because 3/4 of a whole story was about them as teens (around 13 YO).
So I suggest you to read King's Maker first till the last chapter (ch 37,5 prologue of S1), and then come back here to read the sequel so u don't have to reread S2 (both all ages and mature vers)

READ SEASON 1 FIRST DON'T SKIP IT.
If you skipped S1 you'll gonna miss Shin and wolfgang's journey from their teenage days, Shin's childhood trauma, Wolfgang's evolutions from cute noble young master - Kids street gang boss - a Prince, and their cute friendship when they were teens. YES THEY ARE CHILDHOOD FRIENDS CALLING FOR ALL FANS OF CHILDHOOD FRIEND TO LOVER.
Also, there are so many reference to S1 here that if you don't read it you will not know what it means.
READ IT AND IT MAKES YOU FEEL LIKE WATCHING YOUR KIDS/ LIL BROTHERS GROW UP