Depends on gender, I'd say. I can't say for roos, but my pullet's comb started turning practically overnight at about 5-6 weeks. That's just personal experience, mind you. Let's hear from someone else.
As you can see with this 8 week old pair, the cockerels turn red and red wattles appear much, much sooner. The pullets don't generally show much of anything until nearer to maturity. Hatchery birds mature much much faster, with cockerels showing red at 5 to 6 weeks.