Best barrel horse age?

Don't get me wrong, I know a LOT about horses and barrel racing and have been doing it since I was three. My parents didn't own a ranch or anything, but I fell in love when I was about 18 months old and my grandmother would take me to the pony ride ranch by her house. Anyway, that is not my question. I am always really paranoid about a barrel horse's age. I get a good horse, but then get worried and anxious when they turn 10 or so. I don't want them to start to slow down! I don't know why I feel this way. Probably because I know we can't afford board for two horses, and I don't want to fall in love with a horse if I have to sell him a few years later because I need to stay competitive. I have heard it over and over again, 7/11 7/11. But I google people like Martha Josey, Charmayne James, Brittney Pazzi-Phar, etc. and get worried when I see that all their winning horses were like, 6 years old or so. So I was just wondering your guy's opinions on the optimum ago for a barrel horse. Please don't lie to me, be real and tell me what you think with no sugar coated answers. :] Thanks.