Do I want to get into this... why not, "everyone else is doin' it"
Breaking the law: If you are playing and you are under your Country/State's age limit to gamble, you are breaking the law
Breaking the rules: If you are playing and you are under a certain site/B&M's age limit, you are breaking their rules. And in any "game" I've played in, if you break the rules you are "cheating". If you move two squares in checkers and your opponent does not notice, you've still cheated since you've broken the rules.
You can be breaking the law without breaking the rules and you can be breaking the rules without breaking the law. Depends on your age and the restricted age.
It seems pretty simple really.
As for stealing, this is really a gray area, it would take someone in law to clearify if this situation is actually "theft" per say though. But I would tend to agree with Zach. Personally, if someone has "cheated" me out of something, they have "stolen" it from me since they did not take it the correct way.
If a 17 YO played my state's lotto and wind, then cashed the check after they turned 18, technically they stole that money from someone else who could/should have won. Even if it was a $1 or $10M winner, it still should/could have been someone elses. As for the bubble, say said 17 YO purchased that ticket in front of me... that ticket (scratch off since lotto uses RNG) would've been mine to win, so yeah, he TOOK/stole that winner from me. I don't even know if a lotto company would cash said ticket. If this were a B&M and this happened, the B&M would take the chips back immediatly and probably report this player to authorities.
Not only did OP cheat... and yes, I will use the word cheat since he did it against the "rules"; not only did the OP "cheat" the bubble, he cheated ANY player he played against. If he made it to the money he, more than likely, went HU with several others that he knocked out. Not only that, and you're going to love this, he changed how ALL the cards were dealt at any table that he was on. Technically this "cheated" many MANY players since he was there when he shouldn't have been. Want another? If this was a max seat tourney which was maxed out, he cheated 1 "legit" player out of a seat. Plus, are you ready for this one, he changed the seating chart of everyone seated after him, which changes the cards all of those people were dealt. SO, it may be "cool" if someone beats you, but do you realize how much they may have really changed the outcome?
As for something which is considered "accepted" as some of you would "accept" the OP playing underage; Smoking marijuana in the US is VERY common and very easy to come across, but it's still illegal. Should it be legal is a whole other topic, but at this time it IS ILLEGAL. Weather it's "accepted" or not, it is illegal.