I don't know what # user I am (how can I get this information?) but my twitter account was also created in 2007 (on the 8th of April says whendidyoujointwitter.com) and is still active. Also received the email.
The Twitter API exposes a user's ID. Some Twitter clients (like Tweetbot) show this information. You can use http://mytwitterid.com/ to find yours.
Or just view source on twitter.com (after logging in) and search for the first instance of "data-user-id"