Hi, I have some horses in breeding that are also 15 y.o..
Can I use them with best horses or is it a not wise decision?

thanks

I breed to 10 as i usually have 30 to trail with by then and then the best trial performers
go to the barn until 10 again and so on

Personally, I keep them until they die unless I'm hurting for space. I then retire the mares with the worst foals. Stallions I retire based on who has the worst foals and who sires more geldings than colts. I want a solid breeding stock, not a bunch of geldings that are ultimately only time advancement fodder because they do nothing for my bloodlines.

Despite a horse's age, I wouldn't completely discount a 20-year-old mare having a 10+ G1 foal. It's happened in real life and it happens in the game, too. It's up to you how you want to retire horses and why. No one way is really right. If you're hard pressed for space, then you'll have to make some tough choices about who to retire and when. Easier for some to retire at age 10 or 12. Easier for others to evaluate based on foal performance. Others might have a limit of 5 foals regardless of the horse's age. It's up to you.

I had a good stallion live til 30 - he kept breeding good horses.
As said above there is no right or wrong way.

I breed until they get to 10, after which it depends on how they have deteriorated, constitution is usually failing by then and I try to avoid breeding that through, exceptions made for horses with something (speed, burst etc) that I still need to pass on.