On a similar note, most D&D versions that I'm familiar with have high-level options for the creation of private demiplanes with customizable traits; variable time flow is one of those traits. Your dragon could have found or acquired access to a demiplane home where time travels much faster than in the prime material; this could couple well with the "larger hoard = age faster but less secure" thing, since the demiplane would give him a very secure place to put his increased horde while he sits on it and grows.Maybe just state that he was on the age border, and hits his growth spurt into full adult at some point.
Alternatively he goes to the Feywild for a bit and comes back out older as Feywild time is random to how long it's been in the Material Plane.
d20 Result 1–2 Days become minutes 3–6 Days become hours 7–13 No change 14–17 Days become weeks 18–19 Days become months 20 Days become years
And assuming that it survives the dragon's defeat (and the removal of the hoard) in some meaningful way, it could be an interesting reward for the party; a permanent extraplanar base where they can pop off to rest or do crafting or whatnot and come back and only a short amount of time has passed.