At what age is it inappropriate to call people 小李, 小王, 小刘, etc.? And is it different for males and females?
It is not about the person's age, it is about is the person older or younger than you, and what kind of relationship between you and that person.

If someone is about your age, and you two are close friends or at lease having a friendly relationship, it is OK. to call him 小李 or 老李

If someone close to you is much older than you, it is OK. to call him 老李 but not 小李

If someone close to you is much younger than you, it is OK. to call him 小李 but not 老李

Chinese men don't usually call a female friend 老李, 小李, 老王, 小王 .etc.  (it implies she is one of us guys) 

If you are 80 years old, you still can call a 79 years old friend 老李, 小李, 老王, 小王 .etc.