I use a 2 TB drive for Time Machine and have used the disk for about a year or two.
Today I wanted to check on an old file (an old version of MonoDevelop) and found out that my oldest backup is from January this year.
I checked the disk itself and found that it really only contains folders for dates starting in January.
What did Time Machine do to my old backups???
(There was always enough room on the disk and there are 1.4 TB free now. I never deleted anything in the backups.backupdb folder.)