I started gaming at 3(got NES from uncle) and managed to finish Megaman 2, Probotector(Contra) and Super Mario Bros. Although controls were simpler back then but the games were harder and more punishing. Also your cousin is just young and doesn't really quite understand yet so there is nothing you can do there except try and be nice. There a different buttons for Sprint, crouch, prone and stand up so you can move at varying speeds decide how big of a target/accurate and fast moving you wanna be. It is not that arbitrary. One of the biggest things I missed about CoD when trying to play BF:BC2 for the first time was Lean. Yet this is seemingly arbritrary functions serves as a massive tactical advantage than say stand in door way the try move back in when shot at.
As for minimum age there should probably be a minimum age/maturity level for online gaming but that is it really. Well but this is just more to do with the fact that the people shouldn't even be playing the games in the first place anyway.
Just get your cousin to play Mario or Crash(the first 3) or some other game. Try get him to play some simple Flash games on Kongregate but make sure you know what he is playing first as I am sure your Aunt/Uncle wouldn't want their 3 year old playing Last Stand 1/2 or Endless Zombie Rampage or Rage or something like those.
As for minimum age there should probably be a minimum age/maturity level for online gaming but that is it really. Well but this is just more to do with the fact that the people shouldn't even be playing the games in the first place anyway.
Just get your cousin to play Mario or Crash(the first 3) or some other game. Try get him to play some simple Flash games on Kongregate but make sure you know what he is playing first as I am sure your Aunt/Uncle wouldn't want their 3 year old playing Last Stand 1/2 or Endless Zombie Rampage or Rage or something like those.