You may be confusing this with a similar program that CO used to have. In the early 90's, they had a program whereby 5 consecutive years of Gold Elite status got you Lifetime Gold. At the time, Gold was the highest level on CO. This was discontinued sometime in the mid-90's, but people that had started on their way toward lifetime status were grandfathered into the program , and were awarded lifetime status once they got to 5 consecutive years, even though the program was closed to new entrants.

When the CO and NW programs were coupled, the highest tier elite on CO was renamed Platinum, and the Lifetime Golds were made into Lifetime Platinums. So, there are some people flying on NW today that are using their Lifetime Platinum status from CO to get elite benefits, but CO hasn't issued new lifetime memberships for several years.