Buy a house today if you can, but don’t sell one if you don’t have to, says George W. Johnson, a 97-year-old real-estate agent who has been working the Seattle market since 1936.

Johnson, who is reluctant to call himself America’s oldest real-estate agent — he says he just learned of a 99-year-old broker in Florida — has seen his share of housing booms and busts since he hung his first real-estate shingle 74 years ago.