One person I know says it's vital if you want to guarantee a good retrieve and soft mouth... That isn't true, as long as the dog has had the right start. I have working/trialling working Cockers and if I'm buying in a dog I prefer to get them at around 5-6 months old. I have limited spaces so to have the best chance of getting what I need I prefer to see them that bit older so I can see more of their personality and judge levels of drive, focus and trainability. They obviously have to have been in the right kind of environment up to that age though (it's more important what they haven't been allowed to do than how much actual training they've had) and I look for specific lines, it took me a long time to find my bitch and I looked at quite a few pups before I bought her, but as soon as I saw her pedigree and then met her I knew she was the one, I had never seen such focus and drive in a 5 month old puppy. She has been perfect from day one, she's now 3 and is an OFTW, she's one win off FTCh and in over 20 trial runs at novice and open level she's only been out of the awards a handful of times. My older male dog who I bought from his breeder (a gundog trainer) at 5.5 months is a fantastic working dog and my day-in day-out shooting companion, again I was able to get a good measure of him before I bought him.
To the OP it's not too difficult to find an older gundog pup, most good working breeders/trainers should be able to help you. A lot will run on several pups and then sell the ones they don't want to keep themselves, often they will have had at least basic training. If you know the lines you're looking for I would get in touch with the breeders directly and ask if they have any older pups for sale and/or if you go shooting and you've seen dogs you like there ask where they are from.