What determines the age of accountability for salvation?

Many Christians say that if you're not saved by the age of accountability, and you die, you're going to hell...but that if you've not yet reached that age, you're going to Heaven by default. So...100+ years ago, people were marrying at 15-18, having children by 16-20 and life expectancy was around 47 (talking about the US). I can understand then, thinking that someone was mature enough then to have made a decision about his/her eternal soul. In today's world, though, people are in school until their mid-20's, getting married around 30, and have a life expectancy in the 70's. People mature at a much later age now than they did then...so shouldn't that mean that the age of accountability should change, too, and not remain 13 when most kids are still busy being kids and not raised as religiously as they once were? Just wondering. Thanks.