dave2008
Legend
I'm not sure what you're asking here. 91 is the damage for an ancient red dragon; if you're asking whether taking on an ancient red dragon is hopeless for low-level characters, I think the answer is, "Not quite hopeless, but it will feel a lot like fighting Smaug." PCs can be expected to die left and right.
The details will depend upon how "low-level" they are and what their approach is, but at minimum it will be very exciting. Briefly.
If you're asking whether it would be appropriate to boost the adult dragon's damage up to match the ancient's, and then boost ancient's some more--well, I'm not a huge fan of stat inflation. That's not the direction I've gone when boosting my own dragons. An adult red dragon's damage already does as much damage as a 9th level Delayed Blast Fireball that's been charging for four rounds; increasing it further seems excessive to me, and hard to describe in non-numeric terms.
So the direction I've gone instead is to make them cunning and crafty, and give them the tools to be so. They've already got flight and high stealth; even just 5-7 levels of Dragon Sorcerer makes them terrifying (Greater Invisibility, Quickened Dimension Door, Quickened Hold Person IV, etc.) as well as being 100% thematically appropriate.
Imagine if the 17th level party fights their way past the dragon's minions and into his lair, only to find... that the dragon is out? Quick, stuff your pockets full of gold! But uh-oh, he was there all along (invisible and with good Stealth) and if someone doesn't have Perception Expertise or similar . Even if he doesn't get a surprise round, the PCs still have to deal with an invisible, Counterspell-equipped dragon who breathes a huge cone of fire hot enough to vaporize steel (63 damage) and then chucks a Quickened Lightning Bolt for another 8d6 (28) followed by three tail slaps, each strong enough to disable or kill a warhorse, at +14 plus advantage (because invisible) for a total of 51 points of damage. And BTW he's flying, and those tail slaps were made from 15' overhead as he buzzed you.
Alternate scenario: on round one, he drops invisibility to cast Quickened Hold Person IV, and then targets whichever PC failed the save with either (1) Fearsome Presence plus six attacks (claw/claw/bite/tail/tail/tail) for 166 points of damage counting auto-crits; or (2) Fearsome Presence and a grapple/grapple/bite routine on two paralyzed PCs, followed by a withdrawal as far as he can move to isolate the paralyzed PCs from the non-paralyzed ones, and then three more tail attacks for a total average damage of 122 (counting auto-crits) on the paralyzed PCs.
Do the PCs have counterplays available? Yes, they do, and I'd expect them to win the encounter (I hope). They could Counterspell the dragon's Hold Person IV with a higher-level slot and then Counterspell (from a different PC) his attempted Counter-Counterspell. They could cast True Seeing before even entering his lair. They could bring an army of skeletal minions and use them to create a strong point outside the lair and basically dare the dragon to attack them. (Likely to turn into a game of cat-and-mouse, that one; like playing Kriegspiel. Dragon is trying to attrition the skeleton army to death and/or pick off PCs; PCs are trying to use up the dragon's HP/spell points (or slots) and consumable magic items, and they need to do it before it gets too dark.)
But the point is that giving spells to a dragon is more than sufficient to turn it from a relatively straightforward fight into a game of play and counterplay, which in turn helps explain why the PCs are necessary. Sure, an army of eight hundred archers could theoretically kill this dragon if they got the upper hand in broad daylight; but the last three times someone tried something like that, they wound up leading an army of corpses. (Cue story about a dragon Polymorphing into a hummingbird and infiltrating the general's tent; cue another story about a three-day-long living nightmare fighting retreat against a dragon who keeps picking off humans by the score or by the hundred.)
I would generally aim for the monster to have at least one strong counter-play against any obvious strategy for the PCs to employ against him. I wouldn't aim to block counter-counterplay though. D&D is a fun game about monsters and treasure, not a military exercise in contingency planning.
Yikes! Long post - I will have to get back to you later!