Cockatiels live up to and around 30 years of age
Diet can also play a huge factor in feather plucking as well as frustration etc. I would be very suprised if your birds have any kind of mite.
Questions that would really help assess your problems:-
What do they eat?
What size is the cage?
How much out of cage time do they get?
How often do they have human company around them?
Where is the cage normally placed?
If you are thinking of moving them to a window do be careful not to put them in direct sunlight, or at least a place where they can go for shade, otherwise they could get too hot.
Cockatiels love to chew, so lots of chewing toys are thoroughly recommended.
I also wonder is your female might just be going through a heavy moult, have you actually seen her plucking?