When are follower levels determined?

I've only just discovered, almost thirty levels in, that follower leveling works totally different from what I had assumed, and basically I have to reverse my follower strategy. I had assumed that, since my character was approaching level thirty, it was pointless to pick up any follower capped at level thirty because they'd become obsolete too soon. Since level is apparently determined the first time you meet a follower, it's basically the opposite.

So, what interests me is when the level of a follower is set. Is it when you ask them to join you, when you first meet them, or when the character is first spawned? For example, if I walk into Riften and right by Mjoll at around level twelve, and then come back at level twenty five to do her quest, what level would she be at?

On the same note, when/where are housecarls spawned? Do they exist before you become Thane of a hold? Are they created when you first enter the Jarl's longhouse or wherever? Supposing I do all sorts of side quests and skill building and reach level fifty before becoming Thane of, say, Markarth, would that be enough to spawn a top-level housecarl, even if I'd visited Markarth several times before? Or even if I did the quests leading up to becoming Thane at a lower level and just... never spoke to the Jarl about it.

Bottom line, can I get the followers to keep up with my PC, or will I be reduced to luring innocent housecarls into dark alleys and assaulting them with esoteric weapons of insanity?