Kylie and Tyga do have an age difference between them (she's 17 and he's 24), but Kim doesn't care about that, she disapproves of their relationship/friendship because of something else.
According to TMZ, Kim is upset because she is really good friends with Tyga's ex, and baby mama, Blac Chyna. The two called it quits over the summer and obviously Kylie hanging out with him, regardless of what their official status is, puts Kim in a tough spot.
Kim obviously still loves her sister, but she just thinks that both Kylie and Tyga should be handling the whole thing differently.
Given that the Kardashian/Jenner clan is so close, Kim found herself visiting the capitol of 'Awkward City' when Kylie brought Tyga to French Montana's birthday party last night. There were no reports of any drama, but Kim DID crop Tyga out of this Instagram pic.
Guess a picture really is worth a thousand words...