I haven't found any firm definitions on what age "started" is. Anywhere from 2-4 weeks old at a minimum, usually. Some people want started chicks that are past the most fragile age. Some people want started birds that are fully feathered. So, started birds are sold at different ages.
Once they are old enough to be called "point of lay" I would use that term. Some people are looking for a started bird that is just about to lay.
I think your best bet is to mention the age of the chicks, since "started" covers such a wide range of ages.