Different birds can also molt...well...differently. I have a few that I see every year drop their feathers like they're on fire, bald. Then over the next few weeks they just grow them all back at once. I also have others who I will never actually 'notice' molting unless I pick them up and physically check for new feather growth. If normal hours of light and dark are given, all birds should molt.
I have around 700 here now and some are bald for a while and some just shed a feather here or there and grow in new ones. Very private. Mine are fed the same so it has to do with the birds.
Yep, age! I think that most molts before 18 months old are small and pretty un-noticeable. Usually, egg laying doesn't stop with those. Maybe a bird has never had a large molt, but I think that they have all molted somewhat in their lives.