7 mths should be plenty old enough. If you feel there is not enough meat on them then feed them for a week or two on pre boiled oats/wheat/corn/peas/rice etc, and don't let them run around much or fight with each other.
The first day they are confined don't feed till evening, then remove uneaten food after about 15 mins. Then feed 2 or 3 times a day, removing uneaten feed each time. Clean fresh water and sharp grit should be constantly available
Once killed pluck them while still warm, then leave them for a while before gutting. This will depend on the temperature at which they are kept. If in a fridge or the weather is frosty you will need a bit longer, but 3 days should be about right. If it suddenly turns warm take this into account
They should be kept with clean fresh water, but no feed not more than 24 hours before dispatch.
All the best
Sue