That looks like it has potential.
Well, at least the numbers are about right, albeit most age categories for most species have lost a few points of average and potential damage. That might be fixable.
The only real winner in your table is the green, which in the existing design does slightly less damage than the black (but over a larger area, with a more difficult save particularly for fighters).
The big loser is 'Huge' dragons of each species, which only gain at most 3.5 average damage at all age categories and in some cases will gain none. Compare with the existing system of +1 damage per die that makes huge versions of dragons of older age categories very fearsome.
My big complaint is that you have not actually expanded out the table to cover all 10 age categories. I think if you did, you'd begin to see the problem I have with your current numbers - there aren't enough increments per age category. For example, the white starts out at 1d6 and caps at 7d6, which means we are going to have three age categories out of 10 where the age of the dragon goes up and the breath weapon doesn't.
As for the rapid scaling of attack damage at high HD, it serves several purposes. First the power of D&D PC's increases at a faster than linear rate, so damage for dragons intended to face high level PCs needs to scale up to compensate. But, by design, breath weapon damage doesn't scale as quickly as HD does for the larger dragon species to prevent it reaching a point where the breath weapon attacks are die no save events for M-U's and thieves. So the physical attacks have to take up the slack. To prevent them from overwhelming PC's and because it is logically consistent for a big creature not to be able to direct all of its attacks against something rather small, the bigger dragons can't make more than one attack on a PC. But that means to be able to really threaten fighter subclasses, the bite damage needs to scale up really quickly.
You'll notice that in a much less complex way, bite damage on the RAW 1e AD&D dragons also inflates quickly and to fulfill basically the same needs. It's just that bite damage is only determined by species and not age category in the RAW.
UPDATE #2: Thinking about it some more. I do kinda see what you mean though. The chomp of the adult dragons and even to some extent the ancients just doesn't have a lot of bite to it. Rather than attempting to smooth things, I may accelerate up a little faster at first before flattening out and then accelerating up again toward the end. I'll think about.
As for the tail slap, you make a point about me being inconsistent with the 2e tail slap, and I hadn't really thought about how much damage I'd assigned to the tail in a serious manner. In 3e, tail slap damage falls from 2e to be consistent with claw damage. My main motivation in looking at physical damage was to ensure total damage output was distributed across more attacks, particularly to the claws which just didn't do nearly enough damage in 1e RAW. This smooths the damage spikes and ensures the dragon is less likely to 'whiff' a whole round. I personally I'm not really into the idea of the dragon's tail being it's second most powerful weapon. Certainly, it's not its second most iconic weapon. Why, other than precedent, do you want the tail to do more damage than the claws? And, even if I do upgrade the tail damage, would you really want me to make it twice claw damage given how much I've boosted claw damage? At twice claw damage, it would generally exceed bite damage. Do you really think tail attacks are as iconic as the bite and need that much showcase, or do you think that total physical damage is consistently too low?
On thing that is on the list of possibilities is adding a 'Tail Sweep' special attack for higher HD dragons (say starting at 14HD) that lets the dragon simultaneously attack multiple adjacent targets (the number increasing with HD). That increases tail sweep damage overall, without it overshadowing the attention of the claws.
UPDATE: Ok, I've thought a lot about it, and of course you are free to up the damage if you prefer, but I don't think in the minds of most people a dragon's tail is an iconic weapon and therefore people whose preferences aren't set by 2e probably aren't going to feel like the tail isn't doing enough damage. 3e Dragons don't even get a tail attack until they are large sized creatures, and then only a 1d8. I think for the size of creature being represented here, the damage from the tail attack is consistent with the damage you'd expect being struck with a large bludgeoning weapon. Keep in mind that tail is 1 foot long per hit die, so its not I think reasonable to make the tail strike that threatening. I mean Oonga stomping a PC only does 5d4 damage, and Oonga is colossal. The 1e AD&D precedent is only for Taimat's tail strike (or sting), and it only did 1d6 damage.
I do think I will add a Tail Sweep special attack, even if it comes at the cost of a bit of complexity. DMs that don't like complexity just can avoid the option, and it will only show up on the bigger dragons anyway.
That seemed a bit too fiddly though.
Yes. I'm reluctant to add a new table to the description, which means laying the breath weapon damage along side HD would make the most sense. But I'm also reluctant to give up the subtle differences encoded into each dragon by the existing breath weapon design:
White: Weak but good area of effect.
Black: Strong but low area of effect.
Green: Weak, but very wide area of effect and harder to save against.
Blue: Strong, great range, and difficult to resist, but low area of effect.
Red: Both strong and good area of effect.
If we lose the damage based on species and age in favor of damage based on size, some of the distinctiveness above may need to go to maintain balance and prevent it being too clunky.
A new table might be the only way to address problem with increments and distinctiveness.
Alternatively, just add a "Breath Weapons" column besides the natural attacks on the Dragon Damage table and add a note that you add the age category on top of it. So 11 HD Dragons might have, say, "breath 2d10" on the table resulting in an 11 HD Adult's breath weapon doing 2d10+5 damage and an 11 HD Great Wyrm 2d10+10.
Because cross-referencing two different tables and adding them together is really elegant? What do you think you are designing for, Role Master?
