I think I have worked out what you are trying to accomplish:
If the persons age is >= 60 and they are disabled rentmortgage should be 0
If the persons age is >= 60 and they are not disabled rentmortgage should be 458 less
If the person is younger then 60 rentmortgage should have no change.
I may rework this depending on which criteria you feel would be the most common. I am going to guess the last criteria would be the most common. There is not as many people older then 60 then there is less then 60. This would not necessarily be the case if you where writing a solution for AARP.
sixtyandover <> "YES"; RentMortgage;
sixtyandover = "YES" and DisabledQuestion = "YES"; 0;
RentMortgage - 458)
I assume you are computing the deduction amount as you don't have anything in your expression from which to subtract it.
If ( DisabledQuesion = "YES" or sixtyandover = "YES" ; RentMortgage; Min ( 458 ; RentMortgage ) )