So I ended up with a new player who was like level 13 by the time I met him. He was an actual new player and so I decided to help him a bit, give him some new player advice and see how he would do.
Currently, he is level 32 so able to play ranked. However, his gameplay has barely changed since I met him. It still looks like he is almost playing his first games. Maybe he does not want to learn, maybe he ignored people trying to teach him but on the other hand, it can't only be that. When I was level 30 I knew how to play decently and even got silver 4 in my first placements and this was mostly without any help. While he most likely will end iron 4 with ease. I think it partly has to do with the amount of bonus XP you keep getting so you level way faster than your skill level actually improves.
But could there be other reasons? (without completely blaming him completely, because I am sure it's also partly his fault) Since I often see other players around level 20 who play like it is his/her first game on league ever.
And in case there are new players here, how did it feel to you?