Baby Zaiden is 4months!
you can buy one package and try it out since they are only like 3.00. my son uses the medium right now and have no problems. i do use the fast one when giving cereal.

cali
I've seen 2 types of "Y" cut nipples for playtex bottles...I haven't figured out the difference other than the packaging though...one says "Variable" and the other says "Cereal"...either way, we use the one that says "Variable" with formula bottles...we use the drop-ins and I found out that the bag doesn't keep air out with the others...so that's why we use the variable...its basically a solid tip nipple...we haven't had any problems at all with it and if he starts dozing off during a bottle I know the speed of the nipple is still working for him b/c it all depends on how hard he sucks on the nipple...

Treasure Has One, Brewing #2.
Variable goes with the sucking motion, so if baby is sucking slowly, and taking their time, the milk comes slowly, if it's a more rigerous sucking the milk comes out fast. Where as with Slow Flow no matter the sucking it's still a slow flow, as with Fast Flow, the milk is going to drop out even when they are not sucking.