Lots of Potions of Longevity
Potion of Longevity
Potion, very rare
When you drink this potion, your physical age is reduced by 1d6 + 6
years, to a minimum of 13 years. Each time you subsequently drink a
potion of longevity, there is 10 percent cumulative chance that you
instead age by 1d6 + 6 years. Suspended in this amber liquid are a
scorpion's tail, an adder's fang, a dead spider, and a tiny heart
that, against all reason, is still beating. These ingredients vanish
when the potion is opened.
The first few times the dragon drinks the potion, it'll probably get younger. But after the 10th potion, the dragon will be guaranteed to get older every time. Getting enough Potions of Longevity might be tough though, due to them being very rare.
Get a ghost to scare your dragon
Consider the ghost's Horrifying Visage action:
Horrifying Visage. Each non-undead creature within 60 ft. of the ghost
that can see it must succeed on a DC 13 Wisdom saving throw or be
frightened for 1 minute. If the save fails by 5 or more, the target
also ages 1d4 x 10 years. A frightened target can repeat the saving
throw at the end of each of its turns, ending the Frightened condition
on itself on a success. If a target's saving throw is successful or
the effect ends for it, the target is immune to this ghost's
Horrifying Visage for the next 24 hours. The aging effect can be
reversed with a Greater Restoration spell, but only within 24 hours of
If the ghost uses this action, and the dragon's Wisdom saving throw is 8 or below, it will age an average of 25 years. A ghost can try to scare the dragon once every 24 hours. That might not be very quick compared to drinking a bunch of potions, but it's a lot cheaper!