I never have read the Wheel of Time series... It's another one on my to-read list.

Somewhere after Pern and the Deryni and Thomas Covenant...
Honestly, Jareth doesn't really want the title of Lord, however cool I think it would be. He doesn't want to get all the special attention that tends to come with nobles, and he sees it as another connection to his father.
DW.
I think you are confusing Title and Rank.
Title is like Vance's brother been Baron Harkon.
Rank is a lot more complicated and has to do with being heir or younger son. Still Jareth is a noble and is most certainly entitled to a Courtesy title.
Title and Rank come hand in hand. They were each connected to the inheritance, which went to the first son. A second son would only get recognition if his older brother died, without heirs. Courtesy titles were non-existent. We are talking about the Dark Ages, after all.
(If you can show me something (non-fiction, of course) that the second son got something big when his older brother was still alive, and capable of taking the family's estate duties, go ahead and post it/let me know, as it would be much appreciated. Right now, I'm basing it off my old medieval studies and what I've learned with my historical recreation group.)
Jareth would be afforded some stuff growing up, as he would still be cared for as a noble's son, but apart from family connections, he really wouldn't be given anything afterward. On occasion he could get a favor from his brother, but that would not be common.