user temp tables not getting cleaned out from tempdb?

Im seeing what looks like temp tables that are hanging around in tempdb and not getting cleaned out.
This is SQL 2008 

SELECT * FROM tempdb.sys.objects so WHERE name LIKE('#%') AND DATEDIFF(hour,so.create_date,GETDATE()) > 12

Im seeing about 50 or so objects that get returned and they are user_table and some of them have create dates of several days in the past.  Now I know users do not run processes that take that long, so why are these objects still hanging around?

TempDB gets reset after a server restart, but other than that, how long are these things going to hang around?