I left my DS rf until about 11mo, but felt the need to turn him around coz he was looking cramped and kept trying to sit up more. But I had a friend who made her DS ff at 5mo.
WilliamsMummy, I don't think your DS would have been uncomfy, just probably wanting to be sitting up. That's the difference between our RFing seats and European ones. Ours are designed for babies, so laying back. European ERf seats are designed for babies and older children so they can be installed more upright and most have recline options for long trips and sleeping. So by the time they are a bit older they can be sitting more upright. European seats also take up much less room in the car for this reason, they don't need the heavy recline after baby mode IYKWIM