They can breed whenever they think they can.
If you're wondering whether the eggs will be fertile, you can crack eggs open and look to see if they're fertile, or you can put eggs in the incubator and candle them a week or so later to see if they're fertile.
As long as the eggs are fertile, there's nothing to worry about on the rooster's side.
(If the eggs are unusually small, from pullets that just started laying, then you'll get smaller-than-usual chicks when they hatch. I do not know if such chicks will grow up to be normal size adults or not--I've seen conflicting opinions on that.)
As for what age rooster, I've seen accounts of roosters 6-10 WEEKS old siring chicks (rare), and other accounts of roosters that were over 6 MONTHS old and their owners were getting impatient (also rare). For a very rough guideline, figure a rooster is mature and fertile sometime before pullets of his own breed/age will lay eggs.
