bad cat

why does my younger cat beat up my 17 year old cat he pounces on her and bites her back until she screams

[Suzanne Hetts, PhD]

If he's biting her on the back, it sounds like it resembles the way a male cat will grab a female during the mating process.  - I assume your male is neutered but if not he needs to be.
Without seeing your cats I dont' know if this is the case, or if your male is trying to play or being aggressive.  
At any rate, you need to find someone to help you with this because your 17 year old cat shouldn't be harrassed like that - she's too old and that sort of thing is too stressful for her - if it is happening frequently it is decreasing her quality of life and the stress can also affect her health.  
Separate the cats if necessary, or even consider installing an indoor, magnetic cat door to a spare bedroom for example, so that your older cat can wear a collar that releases the magnet.  This will allow her to get into a room and escape your male because he won't be able to follow her.
We have a DVD on Helping Kitties Coexist that also provides LOTS of info about helping cats get along.  You'll find it at www.HelpingKitty.com