I think it depends on the person — I still have two DGM's, both 90.
If one died tomorrow I'd feel it was too soon — she still does a lot, and looks forward to things like seeing us all at Christmas, weddings, she'd like to see a great-grandchild etc. She makes plans for things like birthday presents, and what flowers she wants in her garden next summer etc. The other has dementia and doesn't really know us any more and spend a lot of time upset and scared. For her I'd still be sad she'd gone and miss her, but realistically she isn't enjoying her life or looking forward to things.
In general probably under 75 sounds quite young to me (although I've been lucky I suppose — most of the family I can remember have made 85, and a few have been over 100).