Some breeds mature earlier than others, and their bones are set and capable of taking more exercise than other breeds of the same age ....
Some unfortunate owners take to heart the limited information they are given by their breeders - some time ago we had a lovely 7 month old Great Dane dog admitted to the Centre - he was completely under-socialised however, as his owners had been told not to take him out at all until he was at least 12 months old, and had despaired of trying to keep him in their garage when he was climbing the walls !!! (I loved him to pieces and he is now living very happily with another GD somewhere in Somerset!!).
Generally, the larger the breed, the slower they mature, and the longer it is before they are truly an adult. Conversely, the larger pedigree breeds live for a lesser time than most cross-breeds!!
It is difficult to gauge walking times, especially when you have an older Cocker, with whom your younger pup just can't wait to rag around ... The best thing is not to go for a structured walk (ie from A to
but to go somewhere where they can have a walk and then play, which is quite close to either home or to where you have parked the car!! You can then judge when they (or the little one) has had enough, and if necessary put them on their lead whilst you continue to play with the older one until they are happy to quit!!