You can process at any age or size. Age of the bird will determine cooking method, not weight. Feed and exercise both influence flavor and texture, as will birds getting their hormones as they transition into sexual maturity.
If this is your first batch, why don't you process a couple this weekend, make more room in that cage, get a feel for things, how long it takes, etc. Test out your system, make sure you like your process. Takes some weights.
Put the other birds on pasture. Process in another week or two, take some weights.
Age them both the same, then prepare two birds - one first batch, one second batch, at the same time with some dry heat method. Expect leftovers. Compare. Which did you prefer? Was the weight gain and flavor change on pasture (likely slower than caged feed) worth the extra time and risk?
Only you can answer that.
