Its true though... when i was 5, i could beat nearly any attari game, and almost any NES game.. i was able to beat Contra when it came out (i was 8)
My son, who's 5, can play mario64, spiderman web of shadows, tf2 (no joke), even portal (a little bit) but he can't play the old mario games (nes), and other such games...
which SHOULD be easier...but their not.
I dont' really think its that 8bit side scrollers are easier or harder than modern games, or likewise that modern games are easier or harder than older games... I think its just a skill set you learn based on what you are subjected to. My son grew up around 3d games. so he's good at those.
We (old folks) were raised on 8bit over-head & side scroller games. and grew into the 3d games as they were developed. so we are good at both.