Which is true only if we assume a precise equivalence between the species. If a juvenile dragon is a ravening predator on sapient species, and an adult dragon is a ravening predator on sapient species, then the main difference is in size and power. In which case, yes, they are acceptable targets for PCs.
If a juvenile dragon is playing with blocks and making paper butterflies and going "goo goo," then I'll accept it as an innocuous noncombatant.
If it's a fire-breathing savage which has just dominated its first group of lizardfolk followers, devours the local terrified frogfolk and the occasional fisherman, and is proud and lethal as Lucifer, then I don't care if it's technically juvenile, it's still a legitimate combatant.
D&D clearly assumes the latter. It's fine if Mr. Burlew doesn't in his world, but to portray RAW D&D as encouraging PCs to kill "dragon children" in the sense of "cute harmwess wittle dwagons who wouldn't hurt a fwy" is completely off the mark, IMO.
Again, I'm not arguing against Mr. Burlew's ideas. I'm simply pointing out that the age category concept of dragons in D&D doesn't change their basic ferocity and peril. And that a juvenile elf is a good deal more harmless and helpless than a juvenile dragon, by RAW.