When are follower levels determined?

As @JonathanDrapeau said:

Related answer since patch 1.6, followers have the same level as your character and level up with your character, up to their max level. – Jonathan Drapeau

But to answer your question about Housecarls existence, the wiki page gives all the information you need including this line:

These housecarls won't exist in the game until you become a thane; however if you lose your thane status in their original hold, they will remain and will continue to be available as a follower, and will also continue to address you as thane.

Meaning that until you become thane of that hold, they simply do not exist. If you loose your title, they stay.