I don't know about everyone else, but I used to be substantially better at video games when I was 5-10years old than I am today. The entire Ninja Gaiden NES series, Super Mario 1 & 3 etc. I could run through Ninja Gaiden 1 on just a couple lives back then, no continues. For some reason I have trouble beating the first level now without dying 20 years later.
A game accessible to a five year old, yet easily enjoyable and challenging to adults is indeed a great piece of design. For me, I am 99% sure I would have found this game easier as a five year old than I will as an adult.