They may also get a little feisty when they decide it's time to mate. The cleaning is a sure sign (my pair incessantly cleaned one leaf of my Amazon Sword for days), but even more interesting in my 46g was the fact that no other fish in the tank would swim into "their" third of the tank. They had quickly bullied their way to a lot of peace and quiet.
Not particularly wanting them to breed, I lowered the temperature in the tank a little bit (it was high, about 80) and they calmed down, and now everyone else is allowed into "their" third of the tank again.
To answer more of your original question, yes, they are definitely old enough. Mine are still less than a year old and, in fact, were probably about as old as yours when they started.
One final note: as Dangerdoll said, if you want to try to raise the fry, definitely do some research. Rearing them is certainly possible, but they are not the easiest fry in the world to raise!
Best of luck!