Because there has been much discussion around dragon ages of late, here is my general guide that I tend to use to interpret things. This is not based off a mathematical formula, but rather off interpreting behaviour, so it's probably less sophisticated than other people's might be. Anyway, this is my system, which works for me, and might work for you:
(and yes if you saw this on Reddit, that was me)
First Year Of Life (so 0 years old): 0-5 in human years (and this is where Bumblebee and Auklet are)
Second Year Of Life (1 year old): 5-9 human years (this is where Cliff and arc one Anemone are)
Third Year Of Life (2 years old): 9-12 human years (this is where Mink and Ostrich are, and possibly Anemone by arc two)
Fourth Year of Life (3 years old): 12-14 human years (this is where at least Arc 1 Kinkajou is)
Fifth Year Of Life (4yrs old): 14-15 human years (this is where Moon is, and possibly Kinkajou by Arc 2)
Sixth Year Of Life (5yrs old): 15-16 human years (this is where Qibli, Winter and Turtle are)
Seventh Year Of Life (6yrs old): 16-17 human years (this is where the DoD are in Arc One, as well as the majority of the main Pantalan characters (ex. Swordtail))
Eighth Year Of Life (7yrs old): 17-18 human years (Seven is the Pyrrhian age of majority, and this is where DoD are by arc two, as well as anyone else their age (e.g. Fatespeaker and the alternates, Clay's sibs), plus Hailstorm and Swordtail).
Ninth Year of Life (8years old): 18-19 human years (this is where Peril is)
Tenth Year Of Life (9yrs old): 19-20 (this is where Riptide is, and also how old Katydid was when she had Cricket).
Most* dragons above the age of nine are no longer treated as 'young', and it's noted that ten is the age where NightWings are allowed to leave the island, so we shall assume that once you hit about ten, you're no longer a 'young adult', you're just an adult who isn't considered particularly developmentally different from any other adult dragon outside of how much life experience you have (much like you get to a certain point in your twenties and then you're just An Adult), thus I haven't continued the chart beyond this point.
*this naturally excludes Deathbringer, because Deathbringer's age makes no sense with the rest of of canon. Despite 'Assassin' giving us solid evidence that he is thirteen, this clashes with all other textual evidence given, and frankly, I doubt Tui would have intentionally written such a large age gap into Glorybringer on purpose. For discussions about age, it doesn't make sense to include him.
