So I gave it a day to reset after so many changes and then just did a fresh read of the Red Dragons.
Dragon Scales: So I think the current idea works very well, just throwing out one more alternative idea.
A dragon has 3/4 cover against all ranged attacks (aka +5 to AC).
--The idea here is to allow players that have cover negating abilities to shine against the dragon, or using advantage mechanics to combat the AC boost. In other words, it allows players to overcome the dragon scale ability in other ways than "get close". Flavorwise, this may also better model the notion of "finding weak spots amidst invulnerable scales".
Young: Solid
Juvenile: Solid
Young Adult: I do think this is the weakest red dragon in the series. Its at the CR where you need serious crowd control/magic defenses to start carrying your weight as a legendary creature but doesn't have the toys yet. Might I suggest giving the tail the stun effect of the adult dragon. This gives it a bit of crowd control and makes sense in flavor, as this "tail slam" is now a legendary action and not just a basic one.
Adult: Solid
Elder: Hellish Rebuke snuck back in to the statblock. Also I'm getting a CR 23 for this one (Defensive CR 22, Offensive CR is 22 for damage, but +2 for an additional +4 attack for a CR 24).
Ancient: Solid. I do think the inclusion of the volcanic gases legendary action will likely create a large difference in how an Elder and Ancient fight. Because the volcanic gases has such a good chance of knocking people out of the round, I image you will see 3 uses of this as opposed to detect + tail slam for the elder. This might get a little frustrating for players, having to make 3 poison saves a round or be out of the fight for a round. That said, at these CRs I expect those kinds of shenanigan's and for players to plan against it.
Wyrm: The indefinite madness seems a nice mechanic way to handle the fear boost, I like it.
Dragon Scales: So I think the current idea works very well, just throwing out one more alternative idea.
A dragon has 3/4 cover against all ranged attacks (aka +5 to AC).
--The idea here is to allow players that have cover negating abilities to shine against the dragon, or using advantage mechanics to combat the AC boost. In other words, it allows players to overcome the dragon scale ability in other ways than "get close". Flavorwise, this may also better model the notion of "finding weak spots amidst invulnerable scales".
Young: Solid
Juvenile: Solid
Young Adult: I do think this is the weakest red dragon in the series. Its at the CR where you need serious crowd control/magic defenses to start carrying your weight as a legendary creature but doesn't have the toys yet. Might I suggest giving the tail the stun effect of the adult dragon. This gives it a bit of crowd control and makes sense in flavor, as this "tail slam" is now a legendary action and not just a basic one.
Adult: Solid
Elder: Hellish Rebuke snuck back in to the statblock. Also I'm getting a CR 23 for this one (Defensive CR 22, Offensive CR is 22 for damage, but +2 for an additional +4 attack for a CR 24).
Ancient: Solid. I do think the inclusion of the volcanic gases legendary action will likely create a large difference in how an Elder and Ancient fight. Because the volcanic gases has such a good chance of knocking people out of the round, I image you will see 3 uses of this as opposed to detect + tail slam for the elder. This might get a little frustrating for players, having to make 3 poison saves a round or be out of the fight for a round. That said, at these CRs I expect those kinds of shenanigan's and for players to plan against it.
Wyrm: The indefinite madness seems a nice mechanic way to handle the fear boost, I like it.