Depends on the dragon and the home. You want it to be big enough not to get lost or be able to get into places that are not accessible.
I live in a old house with build in heater vents, the spacing is wide, no way I could ever let a baby or juvenile roam. Now that sam is big enough his head won't fit between the vents, he can finally get out.
Any roaming he has done in the past was leashed
I agree, it really kind of depends on the situation. If you have a safe area that you can close off with no other animals around and nothing along the floor that can be ingested, then I find it to be a good way to offer enrichment. Younger dragons tend to require more close supervision - I wouldn't leave them unattended when out and about. When to start offering this really comes down to their individual personality. How are they with handling? Are they energetic and curious? etc. I would certainly wait until they are big enough to not get through vents or anything like that.