My own carpets I had as adults. I keep a friends hatchling for a few months so that is my only experience with carpets growing. I have grown plenty of other species though. I usually keep hatchlings in 360 square inch tanks, which is close to your 18 cube, and move them to their adult tank when they get too old for that. Weither it's 6, 12 , or 18 months long just use your 18 cube and when it gets too big for it, move it into a 4x2 foot tank. Don't get another one right now to just upgrade it again later. You already have the one so it would be a waste to buy another.
You will know when it is time to upgrade just by looking at the snake. You will eventually notice one day it looks too big for its tank and this when you buy a new one.