I just recently did my first RAW tweaking (stuff from the
Mods you make every game including giving Dragons and GCS CHILD tags. I put both at 50, which apparently was quite accurate for dragons, since I was trying to base it loosely around D&D 3.x edition dragon age categories as well. Putting it somewhere in the middle of Juvenile would also be justified, since I've always understood that as the "teenage" age category - transitioning from child to adult, and sexual maturity can come during that process, not necessarily at the end of it. Of course, I mainly wanted dragons to breed during worldgen (not sure if they actually do, but eh), and for captured, older dragons to be able to breed. It's unlikely I'd play for long enough to have fortress-raised hatchlings actually mature to adults. If you want to raise your own breeding population, then something like [CHILD:10] is more reasonable, if somewhat cheaty(?). At 10 years, a dragon is already about 250 000 cm
3, which is larger than a grizzly bear, or the same as a troll.
Should probably set up scaling bodysizes too if you want to simulate D&D-style dragons. Perhaps even extend their body-size - very old and ancient dragons are positively MASSIVE, and it would mean dragons that make it past their adulthood would become real terrors - imagine a dragon who actually lived through a 1000 year world-gen.
When fully grown at 1000 years old, a dragon is huge, 25 million cm
3, or in other words, the biggest land-dwelling creature in the game. How much larger do you think they should be? AFAIK the ease of killing them is due to other factors; such as not being made out of bronze and so on... give them adamantine scales or something if you want them to be tougher (would also nicely fulfill the "weak point" trope à la The Hobbit and various other sources)
Edit: GCS are adolescents at 1 year old and fully grown at 2, so should probably lower their child value accordingly. whoops.