In the book Protector, Brennan takes Roy on a voyage to intercept the Pak scouts and lead them away from Sol. Throughout the five to ten year voyage Brennan teaches and trains Roy how to handle the ship and the weapons, how to fight Pak vessles, and how to fight a Pak in hand to hand combat. All of these tasks could have been accomplished much more easily had Roy been a protector from the start. Indeed, the training may have been completely unnecessary even.
While Roy might have been too young at the onset of the voyage (I cannot recall his age being mentioned), he was clearly old enough at the end that Brennan infected Roy before he could get to Home.
Why did Brennan wait until just before he died to turn Roy? Was it really just Roy's age that delayed him or was there some other reason?