20 years as SO is absolutely wrong. The longest I ever heard of was 12, most were less and their time as FO was far less than the average.
Most of them then had the benefit of the introduction of preferential bidding, plus an extension of their flying careers way past the age of 55 that was initially envisaged. Couple that with the changes to super that the Howard Govt brought in and they have had a very good run indeed.