To me they stop being green and are “made” when they know their job and don’t need to be taught how to do it anymore. This isn’t to say that a made horse will just cart you around while you do nothing, but in my mind they can do what is expected of them with a competent partner (read: not a sack of potatoes) and without having to be micromanaged the whole way around.
With this as my definition there really isn’t a time frame on when it happens and a horse can be both green and made at the same time depending on what you’re talking about. I come from eventing world so my points of reference are a little different, but for example, my horse is a “made” Starter horse but still green at Beginner Novice as an 8yo (courtesy of some lameness issues that have held us back for most of the last five years). By the end of next show season he’ll ideally be a made BN horse and will be green at Novice, then made at Novice the following season and green at Training, etc. etc. etc. As Trekkie said, an UL event horse may take 5-8 years to be “made” at whichever level they’re topping out at. To me it’s only tied to age insofar as there’s a minimum amount of time required to reach that point for whichever level and you just cannot feasibly get there any sooner.