I have a 2 year old ferret and a 6 month old I been trying to get them to get along but the younger ferret, always licks and nips the older ferret. My older ferret hisses at him and sometimes yelps its not a really loud yelp but I can tell she doesnt like it. I need to know if I should let it go on or just keep them in seperate cages I cant tell if my older ferret Izzy id just annoyed by my younger ferret or if hes hurting her. How can I know?

     Sorry if this doesnt make sense I been up all day doing finals.

A 2yr old and a 6 month old Kit. The Kit has not figured out the RULES yet, what is too rough, when to back off. All ferrets play fight they don't play with their paws they play with their mouths. The general rule is NO POOP, NO BLOOD, NO FOUL! The Yelp his your 2 yr old's way of saying "HEY Easy there" or "BACK OFF" If the Kit respects that and backs down/eases up when the older one yelps then he is getting it. If he still goes full on when Izzy yelps or is seeming aggressive (shaking his head while he is biting, going for the throat) you may have an issue. 
It may be easier to keep up the intros now rather then wait until he is older he is a Kit and you could help train him by interceding when you feel it is too rough (you will know by Izzy's reaction)  
We tried about a dozen times to introduce Captain and Morgan to Pirate. Did not go well ever, Pirate wants nothing more then to eat them, goes right for the neck grabs on and shakes his head (like he is trying to snap the neck of his prey). He has never drawn blood on them but his reactions and body language and their poop said all I needed to hear. He was an only ferret for about 3-4 years, we got him at 6 weeks so for a long time he was the only animal in the house (not counting the fish and turtle that don't interact with him) We got Treasure (Puppy) about a year or so after Pirate even though she outweighs him by 27 lbs HE is the Alpha in the house (aside from wife and myself) so yes a 3lb ferret terrorizes the other 2 ferrets a 30 lb mutt and now a 65 lb collie. Because he was never used to other ferrets he has never accepted them even though their cages are side by side for the past 3-4 years. The dogs well they are just good chew toys Puppy noses, tails, and paws are tasty. 
Be patient and give them time just make sure you supervise all interactions. When you see them start sleeping together and cuddling up w/each other that is a good sign. 
Some have said that they had some luck doing intros by having them eat from the same bowl (favorite treat or duck soup) I tried they were fine while eating but as soon as Pirate finished he would stalk and pounce on them, even tried being sneaky by walking away and behind me to ambush them. 

good luck