actually, i dont think 18 is a good legal age these days when it comes to financial responsibility, because, pretty much 99% of the population doesnt learn the slightest thing about how the world of money works. few people learn how badly a few bad decisions with your new, fresh creditcard can hurt you in the long run, before something bad happens. and those colorful flyers never mention all those negative aspects.
everyone is just thrown into this mess and most are lucky enough to have parents who can help and answer questions until you slowly get the hand out of things. but some people have moron parents. or know only people who never had to worry about this stuff in the first place.
when 18 as a legal age was established where I live, the world was a lot less complex. at 18, you had all the knowledge accumulated to go through life mostly without problems. today, things are different. vital information is kept from children (in favor of useless geography and history/other crap), not included in the school curriculum, adults have problems distinguishing the vital from the nonsense information, and theres a million things diverting the attention of the young, growing mind.
so no, 18 isn't a good age for legal responsibility. it has always been a rather random number to begin with, but it's become a less and less justifiable random number as time moved on. if young people are actively educated on these things prior to the age of 18, then it might be a good age again. but not as things are now.
you however, have to deal with it now, i'm sorry for you. even though, from an objective standpoint, it seems a bit ridiculous you cant open an account at all?
considering the facts that surround your credit rating, you acted rather quickly, got out of your debt fast and responsible. there are people out there who take a lifetime to repay their debts.
as a financial institution, I probably wouldn't give you a no limits account either, but whats the problem with an account with a strict credit limit? like 100$ max or whatever? i mean, if you have an income too?
current credit ratings certainly have broken the system partways at least. I get that a financial institution would want to base its money lending policies on some information, but credit ratings seems awfully skewed towards negatives. there should be an additional rating for getting your act together. personally, I'd rather lend someone money who quickly got his act together after fucking up as a youth, than someone who, in his 30's keeps falling back into the negatives at the end of every month (though thats an entirely different problem altogether)