When do chickens start laying?

[chompie]

We've recently got our first bantams (Buff Orpingtons & Pekins).
They are currently 15 weeks old. When should we expect them to start laying? Is it from about 18 weeks onwards?

We've got sussex something or others and it was about 5 months old. But worth the wait as they've given us an egg each right through the winter - never have so many pancakes been tossed by so many eggcited kids Good luck

Thanks. We can't wait for our first egg!