What suppling exercises are you doing? I think straightness training sort of work really helped my oldie, but generally mixing it up and not doing too much of anything. Are there any particularly stiff bits you are trying to target?
I find the worst thing by far for stiffening up older horses is bringing them in - if you can keep them out so they move about naturally they retain their suppleness much longer.
It's why I sent F back 'home' the last thing he needed was 6 hours turnout a day in winter to start early october. Even though our ground is wet we can keep them out 24/7 till fireworks.
Carrot stretches
I do some targeted in handwork before mounting
I have a massage pad to warm back up
Keeping him warm enough, and good exercise sheets in winter
Working him nearly every day, with a wide variety of work, to within his fitness level
Allowing longer to warm up and seeing how he feels on any given day
Micronized linseed
Regular shoeing, and really careful foot balance
Regular bodywork
Lots of turnout (mine is out 16 hours a day)