Blizz has already told us canonically (at least until they retcon it) how long expansions and the time between them is.
Taken from Wowhead after a forum post by Steve Danuser (narrative director of WoW): -
Year |
Duration |
Expansion |
0 |
|
Opening of the Dark Portal |
0 - 25 |
25 years |
Events of Warcraft 1, 2, and 3 |
25 - 26 |
One year |
World of Warcraft |
26 - 27 |
One year |
Burning Crusade |
27 - 28 |
One year |
Wrath of the Lich King |
28 - 30 |
Two years |
Cataclysm |
30 - 31 |
One year |
Mists of Pandaria |
31 - 32 |
One year |
Warlords of Draenor |
32 - 33 |
One year |
Legion (Four years after the end of WotLK) |
33 - 35 |
Two years |
Battle for Azeroth (Anduin’s age officially stated as 18) |
35 - 37 |
Two Years |
Shadowlands |
37 - 40 |
Three Years |
Time Skip between Shadowlands and Dragonflight |
40+ |
Unknown |
Dragonflight |
So as it stands Anduin is 25 at the start of DF, since he is stated to be 18 after we’re done with the Argus Campaign.
Edit: That is, provided that there’s no temporal shenaniganry going on, as some realms experience time differently, with Turalyon and Alleria experiencing a thousand years with the Army of the Light while only decades passed on Azeroth.