Dragons live to be over 1,000 years old in DnD 5e. That is, as long as no upstart adventurers slay them in battle and lay claim to their vast hoard of treasure.
A dragon’s age is directly tied to its power, as well. From the age of 0-5, a dragon is but a mere wyrmling, a medium creature with a measly 2 to 4 challenge rating.
But if a dragon reaches the ripe old age of 801, they are an ancient dragon, a gargantuan monster with a challenge rating of 20+.
A dragon of who lives a millennia considered a wyrm, while one that celebrate their 1,200th birthday can call themselves a great wyrm.
Dragon Age Categories
|5 years or less
|801 years or more
Monster Manual, pg. 86