I would assume
Kathryn was 20-30. Although I think its dated, it seems to still be well-liked and used often. I would use
Kat,
Kit or
Kitty as NNs. I don't like
Kathy or Kath.
Even Ryn ("ren") would be better.
I prefer
Katherine because its more complete and Y's look a bit odd near N's. I don't necessarily like
Katherine, though. I much prefer
Caterina.
https://youtube.com/@MarioDrama632 or
https://youtube.com/@Objects_2763 :3