Scott was 6 the first time I took him to ER. I made sure we had aisle seats (as I do everytime I take the kids anywhere) so that if he was getting up and down for the toilet, juice etc he wouldn't bother anybody. Its a lot to ask a small child to sit in the freezing cold watching a football game with 10000 adults shouting and (hopefully) cheering.

What age they should go depends on the child themselves. I'd say if you are doing it for you then dont. If you think they would enjoy it and are doing it for them then give it a try. If they dont enjoy it it doesn't mean they never will. Choose the game with a bit of sense. An OF game or a derby isn't the best first experience for a timid 3 year old but could be an amazing experience for an outgoing 6 year old.