Update (1/1/19): My account is back! I've kept this page live just in case anyone is still interested in what happened.
TL;DR: Locked because I wasn't 13 when the account was created.
I had just turned 18 and I noticed that my birth year was still 1998 instead of my real birth year, 2000. Since I was 18, I assumed that there would be no repercussions.
However, since Twitter is literally run by robots and everything is automated, this happened:
I did fill out the "form" that it links to – it was literally just a set of radio buttons for reasons why your account should be unlocked. I selected the "I am now over 13" option (or whatever it said) and haven't received a response from them yet.
So I have made a counter to keep track of how long it's been since I submitted the form:
DAYS SINCE ACCOUNT LOCK
I also re-purposed my @tjhornerbot to tweet @Twitter and @TwitterSupport every day with how many days it has been since my account was locked. For example: