discus age

hi every one ;
i have been looking after discus for the past 5 years and i have had my fair share of ups and down with this beautiful fish , lately i have lost a few fish which i have had for 3 to 4 years for unknown reasons , as an example i had a stenker which i got when she was about 12 month and i lost her to some bacterial infection 3 month ago or a Marlboro red which i have had since 8 weeks old and has fin rut and very skinny and although still alive i am not holding very much for . i have had him for 4 years .
The question i have is how long your average discus live in your care .
i have red in various forums that discus lives up to 10 years in captivity but in my recent experience it seems to me that after 3 to 4 years the immune system of my discus is reduced i.e they secum to some kind of infection now is this as the result of 1 . old age or 2 .the way i have kept them in my care , i have to mention that with a few exception they all have been locally bred . I will appreciate if every one says how long their oldest discus lived and how they passed away.
regards
sayid