The only exception to the "18 months old" rule pertained to holiday releases (for example, Christmas albums).
Many consistent sellers made return trips to the Top Pop Catalog Albums chart each November through January.
The "total weeks" longevity record (by a large margin) is held by Pink Floyd's The Dark Side of the Moon, which has a cumulative total of over 1,600 chart weeks (more than 31 years).
Its closest competitor is Bob Marley's Legend, at more than 975 weeks.
Three of Michael Jackson's albums (Number Ones, The Essential Michael Jackson and Thriller) claimed positions 1-3 respectively on both the Top Pop Catalog Albums and Top Comprehensive Albums charts in the week following Jackson's death.