Metroid was flawed but not what I'd call hard. Yes, it was an exercise in frustration and I didn't bother to play very far (come on, I have better things to do than try 50 times to get through an area at 30 health or spending two hours killing random enemies to fill up my healh beforehand), but the repetitive level design and said health flaw (together with enemies your standard armament can hardly reach or destroy) made playing it feel like a chore. I mean, having three doors that lead to three rooms that look exactly the same and having to try out which one leads to anything just isn't fun.
Yes, kids back then tried their hands at the really hard games, but that doesn't mean they succeeded. Sure, if you only have two games for your NES there's not much to play and you have enough time to master them, but most kids never completed their old games. Hell, when I was a little kid my most played games were Katakis (an R-Type rip-off by Factor 5) and Giana Sisters (a Super Mario Bros rip-off (I even had a hacked version that replaces the player character with Mario), if my information is correct that one was made by Factor 5, too), both pretty long and hard games which I never played past the first few levels wthout using the trainer the previous owner patched in. I never completed a single level in Mega Man for the Game Boy without using an Action Replay (I think I made it to cutman and elecman without help but never managed to beat those). My friends were even worse at games.
The SNES is a completely different story. I made it through most of the games I had for that, mostly because of the implementation of save features and other helps that kept you from having to replay the entire game.
What I wanted to say is: Not even 5% of the kids BACK THEN could tackle the old games. How old were you guys when you played them, anyway? I was maybe six, at that age playing even the easiest videogames like Mario Land (30 minutes start to finish) was really hard.
But then the first GB game I played (didn't like Tetris, still don't) was R-Type...