When I was about 14 I had an account but set it up in my moms name and details, was selling under it. I had some woman who was dissatisfied with something throw a royal shit fit over a $1 toy, who reported me to ebay as being underage, and my account was frozen.
How the hell she 'proved' me being underage, I have no clue, as like I said, my mom's details were the only ones ever given, and the only reference she could've drawn my age from were me saying that my ride to the post office was unavailable on Saturdays. There are certainly people who are over 18 who still need rides, but that didn't stop them from giving me the boot.
That being said, no serious consequences. Just lost my account. I know plenty of underage people (Including my 9 year old nephew) who have their own ebay account with no problems.