There is an element of habit from an early age, but I would argue that it is not the biggest factor. We only fast for one month out of 12 a year, the habit is broken for the other 11. If it was fasting day in day out from a young age then yes habit would be a big influence.
At the same time it's not random, meaning your brain knows very clearly from the get go that this is something that will be done 1) every year 2) for 30 days 3) based on moon sighting.
IMO this aids as opposed to random unpredictable nature where I pick up something, drop it then try to come back to it one day.
Additionally, I'd argue that every time I pray I am somewhat reaffirming my faith and thus fasting that is to come later in the year. Premise here that fasting becomes easy because your body is mentally prepared from years of programming and the constant reenforcing of my religious values when I'm not fasting. Remove those two elements(faith + practice) and fasting will be very difficult.
I have lots of will power in general but limited self-control. Yet fasting is a piece of cake. It wasn't always like it. I remember when I initially started around the age of 9 I would fast till noon instead of sunset. Then gradually by the time I was 11 I could fast till sunset.
I've read in several places about monks and their years of meditating literally changes the circuits in the brain giving them hyper self-control. Check out the book Evolve Your Brain.