Yup, starting them later is the best way to go. Congrats on that, that's extremely rare in the quarter horse world! My quarter horse was started at 18 months, I bought him when he was 8 and he had already popped two splints. He started to break down around 13 and now coming 21 is just pasture and very light riding sound, despite my meticulous maintenance for the past 13 years trying to fix others mistakes. A few years ago he and I did make a name for ourselves in the local barrel racing circuit but the list of soundness issues is about a mile long . . . .
So with a horse of your size and caliber, and sound, he'd probably fetch a pretty penny whenever you decide to sell him : )
I've spent most of my life riding horses. The rest I've just wasted.