Really? I finished Super Mario World as a kid in third grade, and probably got through the secret world (after Star Road) with a lot of practice. I find practically every platformer to be easy enough, aside from Ghosts 'N' Goblins. I recently played Super Alfred Chicken for SNES, and although it's considered difficult I never had too much trouble. By the time I cleared it (granted I got the bad ending) I have around 50 extra men remaining.
As a 14 year old.I fiund NSMB Wii to be pathetically easy. I finished the damn thing (without all the star coins) 3 days after I got it, with maybe 97 lives stockpiled and not even coming close to activating Super Guide. A week later, I got all the star coins and extra routes. I know you don't believe me, but I ALWAYS played games for hours on end as a kid- and most of them being platformers I know all of the tricks they can throw at you.
When I was in sixth grade I got that TMNT game for NES. The water level got me stumped for maybe a day but then I got all the way to the interior of the Technodrome (still haven't beat that part) by a week. That's probably because I found a map of the water level on Gamefaqs and of course all you had back then was a friend to tell you or Nintendo Power.
When I first started playing Mario as a seven year old (beginning with SMB3), I was PWNED by every enemy when I would boot it up. Nowdays, I can beat any of them very easily.