Frazzled, you want to dig out
WD 127 or the Warhammer 40,000 Compilation, if you can.

The Young King is an Exarch who is chosen to be the sacrifice to awaken the Avatar. In the story in that
WD, the Young King is indeed a "rookie" Exarch - obviously a veteran warrior, but only newly made an Exarch (earlier in the story, his friend/lover argues with him, trying to prevent him from becoming an Exarch), but it's not clear if that's always the case, or just in this instance.
The Lexicanum article cites an
OOP Apocalypse datasheet and says it's a position specific to Biel-Tan, but I'm not sure about that. The "Court of the Young King"
formation (i.e the Avatar and a retinue of Exarchs) is a Biel-Tan specialty.
Early Eldar background has fairly heavy celtic* inflluences (such as the names of the craftworlds Biel-tan and Saim-Hann), and the sacrifice of the Young King hints back to the idea of the sacrificial king (as does Terry Pratchett's
Hogfather).
* Some form of European pre-Roman, pre-Christian society, anyway.