Caravels are definitely only available in high and late. There is a 'compass rose invented' message that also is supposed to enable their construction. This, IIRC, occurs about 1170, but regardless of whether you tech up to a Dockyard or not, you won't be able to build Caravels until 1205.

I have never tried modding caravels to all periods and then seeing if the compass invention in 1170 enables their construction with, immediate effect. I'd imagine that, as is the case with gunpowder and it's dependent units, that it does.

Caravels were probably restricted to high and late for game balance issues. They are easily the best ship in the game in terms of attack, defense, speed, range cost and support cost, which are all balanced, to make the Caravel a workhorse ship that you probably won't need to upgrade further.