Accounts which were created before the user.user_registration field existed (before rSVN12207) do not have that data in the database. Accounts without the data had the first edit time filled in as an approximation, but this is sometimes wildly inaccurate. In addition, the field remains NULL for accounts without any edits.
There are several months worth of data on Wikimedia wikis in the new user log from the extension (see rSVN10573 ) that could populate this data. Also, for users prior to even the extension, a Gaussian curve could be plotted from the data of available edits and log entries (all of which would be after the creation date) and normalized to a curve or wave of user creation date/ID.