Will wooden fencing "age" inside a dark (but ventilated) garage?
Since a significant portion of the fence panel aging is due to sun, rain, wind and temperature changes the best way to get your unmounted panels to age is to put them into the same environment as the mounted panels.

This really would mean propping them up so that they are exposed to the elements and the sun. If this is not practical then your next best bet is to get some help and finish the fence construction this weekend.