Some of my bantam Cochins start laying at 5 months, others may wait until 6 months old to start. My standards start around 6 months but I have had them wait until 8 months, too.
My bantams show good promise at 4 months but I have not had many to top out their full mature weight until 18 months. Looking back on my records it appears 16-18 months has always been the heaviest point in their life. That is usually a rest period from first laying and breeding. After that they get really serious into being flock birds and raising the families. Before that they are having fun and games. Some of my girls may chunk up at 2 years old but only for about a month and then they decrease some. Probably the winter feed I put into them then.
My standards can be heavy at a year old and stay fairly heavy through each winter. They are thick feathered birds and the roo is very laid back. They get too hot in the summer and slack off on weight some.