Thing is, if you were truly playing your long lived dragons highly efficiently, then your PC's should never, ever win. There's just too many things the dragon can do. Again, fighting at night means your archers can't hit anything. They are firing blind. I disagree with [MENTION=6787650]Hemlock[/MENTION]'s point about skeletons, since once the dragon gets out of 60 foot range, they don't just have disadvantage, they can only attack by picking squares. Good luck with that. Summoned elemental dies pretty quickly, so, following it around with a light source doesn't work either. Ancient dragons with line breaths have 120 foot range with 120 foot dark vision. Nothing will ever get close enough to illuminate it.
Again, killing PC's is ludicrously easy. The idea that I have to go up CR+5 or 6 encounters to do it is a joke. I can kill PC's quite easily with regular encounters, thank you very much. Been doing it for years. If the DM is actually out to play competitively with the players, the DM will always win.
I don't run attacking in the dark the way you do ("pick a square"). In a game which did, if the party from #284 is attacking in the dark, the simplest and most straightforward way to deal with the problem is to give a torch to several of those 16 Owls (5 ought to be plenty) and give them the job of patrolling above and closing with (thus marking for missile fire) the dragon when it approaches. If any owl ever gets within 40 feet of an Owl, it's lit by dim light, which means it gets hit by the readied actions of all the skeletons. They'll have advantage from being invisible to it, and probably disadvantage from long range (since they're distributed over a 150' by 50' area--obviously you could tweak the area to be larger, and at night larger is more advantageous, but post #284 was showing only how it's impossible for the dragon to win even against a brain-dead approach, so let's just go with my initial 150' by 50'), so they do their full
67.5 damage per round as outlined in #284. The owls have Fly 60 and 120' darkvision, just like the dragon, and they can Ready an action to Dash at the dragon as soon as it approaches within 120'. That drops the distance between them to 60'. It's faster than them, just barely, so it can keep them from getting to 40', but only as long as it doesn't come too deep within the patrol radius, and as long as the Air Elemental (Fly 90) doesn't catch up. How willing do you think an adult red dragon is going to be to run away from the mere sight of a single owl? If it so much as breathes fire the skeletons know its location and hit it (DM's call whether that's disadvantage or regular for both being unseen by each other--you're DMing this hypothetical scenario so let's assume it's disadvantage, so I only do 19.13 damage). It kills 1/16 of the Owls, not even the PCs, at the cost of its breath weapon and a minimum of 19.13 damage. How does anyone imagine this is going to end well for the dragon?
Lair actions are a complete non-factor. Skeletons are immune to poison, and if you prone them they just stand right back up and shoot you, so the only lair action which matters at all is the magma (6d6 = 21 damage in a five-foot radius) which will hit maybe two skeletons (they're dispersed over a 150' by 50' radius, so you're hitting (pi * 5^2) / (150 * 50) = just over 1% of the area and you'll be lucky to catch skeletons together). If the skeletons both fail their saves they'll take about 21 points of damage, but since they have 36 HP each (13 base + 9 from Create Thralls + 16 temp HP from Inspired Leader, granted prior to a short rest) you'll need to hit them twice to kill one, which can't happen on consecutive rounds so it will take the dragon at least three rounds to kill two skeletons (even if they don't disperse from each other, otherwise you only hit one), by which time the dragon itself will be dead because it had to come within 120' of the skeletons to hit them, which means an owl marked it and it's taking somewhere in the neighborhood of 100 to 191 HP of damage (depending on how many owls were able to attack it this turn and whether it ate any opportunity attacks). In fact it could be taking somewhat more than that because now all the PCs with Spell Sniper get to attack it at advantage because it can't see them, but let's handwave that for now and say it's "only" 100 damage per round. It's still not going to kill even a single skeleton before dying.
Furthermore, since we're playing at night, the PCs themselves will not only be dispersed, they'll be sneaking (as best they can with a huge group of skeletons tagging along) and so will the owls. PCs will have Pass Without Trace up from the Shadow Monk, and the dragon has disadvantage to perceive them, so odds are reasonably good that even the paladin with Dex 11 can beat the dragon's 18 Perception-at-disadvantage. (He needs to roll Stealth 19 total, and he's got +15 total because his Dex is odd--under RAW he'd have +14--and the bard has Enhanced Ability (Dexterity) on him to cancel out heavy armor disadvantage, just so he doesn't feel left out. So even the paladin has an 80% chance of sneaking up on the dragon, because Pass Without Trace rocks.) The dragon will know where the owls with torches are, and it will probably know where most of the skeletons are (they have only a 15% chance to beat its perception) and even where most of the owls are (25% chance to beat dragon's perception), but the PCs will be a lurking knife in the darkness.
And that's kind of the point, for me.
Risk is so easy to calculate that rolling dice just isn't interesting. What's interesting is uncertainty, not knowing what the odds are because you don't have all the information, so you don't know what preparations to make. In this case the risk is so high for the dragon that it might as well just give up and die or something (flee to a new homeland?), irrespective of the uncertainty. I think it is more fun to give the dragons sorcerer levels, not least because that inflicts uncertainty on the PCs. What spells does it have? How will it use them? (Shield + Blur is a hard counter for skeleton archers and Owls alike.)
But a regular dragon is toast against an 11th level party, or at least against this 11th level party, even without them doing any fancy tricks. I don't think I'm playing them particularly smart, these are just the obvious things for a party to do when it's built to be well-rounded against all kinds of threats in a game which implements Bounded Accuracy. Leverage ranged firepower, stay dispersed but within mutual support range, make sure you have multiple sources of summoned meat shields, use skeletons because they don't impact the Concentration economy, use your Concentration effectively, use stealth as much as possible, deny your enemy information. I'm sure [MENTION=6790472]vandaexpress[/MENTION] and other tactically-astute players do similar things in their game.
