Hi,
If you have a look in the rescue section you will be able to see a selection of the range of cockers that go into rescue. Some are young 8-months to 2 years, but quite a few seem to be around 3-4 from what I have seen, plus then a few oldies.
There are lots of people who have rescue cockers on here, who will be able to provide you with stories of their dogs.
It may be worth contacting the Cocker breed rescue co-ordinator for your area.
I don't have a rescue cocker, but foster any breed of dog for Four Paws Animal Rescue. So far I have fostered a puppy and an 18-24 month old, I definitely prefer the older dog to the puppy experience. The youngest dog at our rescue is a lovely 10 week old Black Spaniel x called Hattie, then they go any age up from there. It will just depend what is in your area,
Em