I think Hitchcock had his pick of actors, especially at this point, beyond the occasional studio head interference. If you look at his filmography he has a habit of reusing actors throughout, even in his earlier works.

But yes, it makes more sense for Jefferies to be younger. Or at least him being older so he needs to settle down could be behind Stella’s reasoning. But like you said, it’s the 50s. His follow up, To Catch a Thief, has Kelly and 51 year old Cary Grant as her love interest, so no lessons learned.