I was 21-years-old a decade ago. I don't think I've ever put anything excessively offensive or inflammatory into an email, but I'm sure I'd be embarrassed by some minor immaturity issues. Can't think of anything specific, but I'm sure if I reread my emails, I'd find myself thinking "wow, that prof must have thought I was a moron" more than once (if we're including school emails as well as work emails).
Once I was in the working world, I don't think I ever put anything too bad into an email. My team at work is small, so it's easy to just get up and talk to someone face-to-face if I don't want a record of our conversation. If I want to talk to someone outside of the company, I use the phone if I don't want a record, then just shoot off an email confirming any points that I actually do want a record of.
Having entered the working world after email had been around for a while, I've always had a pretty keen awareness of the public nature of that method of communication.