So the UESP Wiki states the following:
A follower's stats are determined based on the level the player is at when he or she first enters the zone that the follower is in. Once generated, the follower's stats do not increase at all. For example, Lydia is usually one of the first followers players will have access to, and most players receive her at a lower level. As such, most players' Lydias will have low stats for the entire game.

My question then is what a "zone" is? Is it the whole city? The reason i ask is because i would like to set up a new base in a town, after having been in whiterun but unfortunately lost Lydia to some falmer poison. I would like to keep the house carl in that town as my follower, but i would also like the follower to be viable to keep for a long time. This means i dont want one that is already locked on a low level. Preferably i'd like to set up in Solitude, but i have already been there albeit briefly, so what i'd like to know is how the zone is defined. Markarth i visited even more briefly, only got there from the "Night to remember quest" and headed out of town as fast as i could.

So would my visists to those towns mean the housecarls there are locked to the level i had when visiting? Or does anyone know the mechanics of this?

I would also like to know the definition to this.

typically a 'zone' in TES games is anywhere there's a loading screen. so, a city would be a zone but there are zones within that zone. so in response to your question, i'm not exactly sure if it means when you first enter the city or if the follower is in pub/inn when you first enter that building.

Quite possibly then the zone for a housecarl would be the castle of the jarl.. But hard to know for sure.