I was just wondering about a few things like what are their last names? When are their birthdays? Even though they're all sixteen, who's the oldest? (At least by days, weeks or months), etc.

Nobody knows, not even the producers.

However, you have 22/365 to get ther e birthdays, and everyone is 16.

But it seems that Trent seems to be the oldest, only because he's more mature then everyone else.