Honestly can’t think of a good reason why Impa and Robbie wouldn’t take Purah’s rejuvenation treatment now that it’s been debugged. Anyone else guessing one or both will be turning young at least temporarily in the inevitable DLC for a companion quest chain or maybe a gag scene?

Probably they don't for personal reasons. Purah seems to just fit the stereotype of the woman who is obsessed with youth and does anything she can to feel young again. Meanwhile Robbie's obsession seems to just explore and create cool shit which doesn't seem to be hampered by his old age at all so he probably doesn't see a reason to de-age. Impa could simply be above such things, she finds grace in her age and wisdom and thus probably doesn't give the whole thing a thought