I'd be fine with same enemies of different power levels if there were visual differences between them.
An example that's been discussed is Young (CR 10), Adult (CR 17), and Ancient (CR 24) Red Dragons. In reality, these are all the same monsters: a Red Dragon. The only difference is age. I could easily see adding a couple more categories between those listed (Baby, Young, Mature, Adult, Elder, Ancient) for CRs 3, 6, 10, 14, 17, and 24.
Give a 10-year old Young Dragon (adjusted to be CR~6) less spikes, make it smaller, give it a less intense color of red along with its lowered stats.
Give an 70-year old Mature Dragon (CR 10, same stats as MHB's Young Dragon) bigger wings, sharper features, and change it's name to "Mature Young Dragon" or "Adolescent Dragon" or something like that.
For humans, it is much harder to make humans look different, especially since human power doesn't perfectly scale with age. A 30-year old human can easily be a Commoner or a Level 20 Barbarian. For humans, titles are much more important and I agree that we shouldn't find level 4 and 18 "Archdruids"
