Soft Focus
My husband and I watched this film last night. We couldn't help noticing that every time they showed Katharine Hepburn, it was shot in soft focus. Even when they showed closeups of Hepburn's and Tracy's faces together, her face would be in soft focus, while his would be normal. It seems the filmmakers wanted to make sure she looked young and beautiful. Personally, I found it distractingly obvious that this was what they were trying to do.
Besides, I think it is obvious that both actors were really a little too old to be playing young newlyweds (not that it's impossible for older people to be newlyweds, obviously). I'm just saying that for that time and place, people usually married for the first time when they were much younger.
I don't think it really matters that they were too old for their roles, but it was annoying that the filmmakers thought it mattered enough that they should try to make her look younger.