Hi Allison
I think it depends on a few factors;
*Is your Lo bored with the existing flow? A sign of this is that they take ages to take a very minimal amout; they get bored sucking and give up.
*The bottle manufacturer. With some manufacturers, e.g. AVENT, Chicco (there are many more...) there is a huge difference in the flow flows; slow flow is very slow and only appropriate for newborn. Whereas other manufacturers their slow flow is maybe a bit bigger so LO's can use it longer.
*Your LO may not get bored with the flow speeds but just have a preference for a slow, medium or fast flow teat.
Many of problems of LO suddenly not taking as much milk as they used to, as recified with a faster flow.
They other thing is I NEVER went by the age recommendatons on the packet, I always follows DS's cues, to gauge if it was too fast/slow or just right. So don't push it if he isn't ready.