IM/GM first steps

How strong would you guess that a typical IM/GM was within a month of learning the moves? My own guess would be 1400-1500. 

Does this mean that if you don't achieve this you are unlikely to ever reach such heights? I would say so personally.

I do not think so. Many later grandmasters stayed weak in the months after learning the game. Most grandmasters learned the game at an early age e.g. 4 years old. The difference between a later grandmaster and an average player is not in how fast they got beyond 1500, but rather the persistence and dedication to sustain progress up to 2500.

It would be interesting to look at a few IM/GM progress charts to see what the typical early steps were. I can't find a resource that goes back far enough even for the likes of Alireza and Pragg... - which would be pretty meaningless anyway because they are truly exception. 

The strongest youngster I have ever played in a League match is up-and-coming Jonah Willow who is now an FM and will almost certainly become a GM (he has a plus score against Peter Svidler!). He started playing in 2012 aged 9.

His progress might be interesting. The early grades are ECF (English Chess Federation) which recently changed to FIDE....  FIDE = ECF x 0.75 + 700

You have to look into the national rating of the young grandmaster to be. Once he gets a FIDE rating it is far above 1500.

I was 800 when I started. But I'll still be GM one

day

I didn't learn how to play until I was 15 (due to Fischer / Spassky mania) which, as with music, second language etc., is probably too late to reach any meaningful height.

Fortunately, chess can be enjoyed at every level!

Yes, I was 800 when I was 15 and now I'm 16, so I assume it's possible

Yes... lucky for us, you don't need to be Bobby Fischer to enjoy the game.