When i come to breeding I notice after the yrs go on my breeding stock horses potential in both codes goes down.
Does this matter when coming to breeding off them?
Surely it shouldn't right? doesn't matter what age the horse is its genes don't change if its 6 or 18 or am i wrong?

Any help guys for future breeding? can i keep a stock horse and breed off it foreever or do i have to change at a certain age.

I haven't noticed this....(not saying it doesn't happen) so I am interested to hear what others in the community say.
Is there an optimal age bracket to breed with?

I think there is, you notice the quality of the offspring really decreases after their potential drops that much. It depends on each horse. But I tend to get rid of them when their offspring aren't as good anymore.

It certainly varies but I wouldn't completely discount older horses. Back in SO5, many here swore up and down that older horses were terrible for breeding and needed to be retired. The best mare I ever had in that game was out of a mare who was nearly 20 and a similarly aged stallion. Unfortunately, I lost her files after a reformat, but she has something like 60 G1 wins and never lost a race. Her dam only had around 15 as did her sire. I haven't really gotten into a game yet for SO6 to comment much on the breeding. I'm working on making my own schedule but keep putzing around on it before I get back into the game.

If you have the space in your barn, just keep older horses until they die. If the foal quality drops, at least you can make some money off them at auction. Making 200k or even a million off a colt you know is a dud is a good way to make some easy money. If can even export the duds into another game just to sell for easy cash early. If you lack the space, then retire the horses with the worst foals followed by the oldest.