are you sure you were 6? I ask because there are lots of things that I remember doing at young ages that actually happened 2 or 3 years later. I can remember getting my face beat in by MM2 when I was 9, but by the time I was 12 I could handle it after a lot of practice. obviously others may be better than I was, but it wasn't until I was older that I could handle those games.
anyway, both of my kids can handle 3d games, but the idea of sidescrollers is actually really foreign to them. they tend to hold a direction for a long period of time because that's what they usually do, so when they try to play a 2d Mario or whatever, they have to fight against that. that's my perception of it anyway. my son is reaaaaaalllly bad at even the first level of Mario World but can beat Mario Odyssey (using the assist mode, but still) and is at the end of Breath of the Wild.
Oh that's interesting regarding 2D vs. 3D games for kids nowadays. Hadn't thought of that. Interesting that they're good at one but not the other.
Also it's possible that I'm remembering from a later time, but I think we sold our NES by the time I was 7. My memories of that time aren't super sharp though, naturally. ---