OMG, this thread reminds me of the day when I was first caled "Ma'am" instead of Miss. I came to Hermes scarves when I was still a Miss, and was definitely a Ma'am when I got my first Hermes bag. That qualified me for Box Calf immediately, with Ostrich, Lizard and Crocodile following not long afterward.
For me, it isn't a matter of age but of style and appreciation. It would kill me to see a young (middle aged or old) woman slip a paper bag with a MacDonald's hamburger into her Clemence Birkin. If that same young woman were wearing an Alligator Kelly Ado (backpack), and keeping the burger away from her Kelly Ado, then it would be a great, modern look. A Lizard or Croc JPG Kelly, especially in a fun color, would be terrific on a young woman, too.
Where I draw the line is what I'll call "bag abuse," either from ignorance or disdain. When I'm in Greenwich, CT, I often see Birkins that look like a wreck -- as if there's some cachet in appearing to completely disregard the bag. Some look like they've been run over. This lack of respect bothers me, regardless of the age of the woman carrying the bag.
I was in Hermes Madison yesterday and saw several Birkins in dire need of Claude's TLC, but the bags weren't being dropped off. Actually, come to think of it, I did overhear one woman asking Claude if he could restore her bag's shape (currently pretty much flat with huge batwings). He sighed and said perhaps if she would stuff it and store it on its back it might help.
The idea of seeing a young woman incorporate a Box Kelly into her look sounds wonderful to me. That look would not be a midriff baring top, short shorts and flip flops. Lindsay, put that Kelly down and back away slowly.