Well, like the title says: Is the AS from minions and/or the jungle creeps affected by the 25% reduced AS? Or does it only affect enemy champions?
If you take a look at this thread on the LoL page they discuss this matter. It would appear that yes it does affect minion and jungle attack speed. Also, if you look at the description of the Unique on Frozen Heart it says "Unique: Nearby enemies suffer −20% attack speed. (1000 range)" Any item that only effects Champions will say it specifically such as Aegis of the Legion.
Aegis of the Legion actually used to affect minions, but they removed that because it was kind of OP. Think about Yorick's minions with the Aegis buff! Terrifying.