You would have lost a certain pool of buyers after your flat turns 40 years old. That's because buyers can't use their cpf for the flat after it is 70 years old. For flats that are in between 40-70 years old (meaning remaining lease of 60-30 years), buyer's age plus remaining lease must be at least 80 years for them to still be able to use cpf and/or take max loan. Or else they have to buy your flat using more cash. Unless your house is in a prime district and many people are willing to spend cash, I would say your flat loses value after 40 years.