Older cats never appreciate having younger ones around, especially when they are at that age. The older cat is sort of trapped between two behaviors, the "It'* a kitten, better not mess with it or its' Momma is gonna whup my butt" and the, "Another cat in my territory?!?! I must whup its' butt, and thereby establish my dominance!"
Make sure you give just as much attention to the older cat, and keep an eye on it to make sure it doesn't ignore surrender posture from the youngster. If it does keep attacking after the kitten squawks, then it'* time to step in and re-inforce the fact that YOU are the dominant cat in the house.
Don't worry too much, they'll sort it out soon enough. Probably be a good idea to wear long pants until then, a roiling boiling catfight to the back of the leg isn't a good way to interrupt your lunch.
"I have a hammer! I can put things together! I can knock things apart! I can alter my environment at will and make an incredible din all the while! Ah, it'* great to be male!"