I'm not sure about the exact age of my three BA's but I got them at "point of lay" around 18 - 24 weeks old, and one started laying 6 weeks later. Still waiting for the other two to start. One looks ready, the other still looks immature.
Florin, our one who is laying only started earlier this week but has given us 5 eggs already, and they range in size from 48g - 55g and all have been perfectly formed.