As goalie and goalie parent -- I offer my 2 cents on this:
It is so true that goalie development is a marathon and not a sprint. What you think is a good goalie at u10 and what a good goalie is at u16 or u20 are so far apart its not productive to even discuss in this thread -- they are 2 completely different animals. Try not to judge skill when they are little and learning.
As to when they show true skill, I would opine that u12-u14 is where you see whether a goalie has the chops to be competitive at the next level. It is no coincidence that this corresponds to puberty.
Please, Please, Please make make sure they skate out as much as possible -- even if in full pads. Pure athleticism is often enough to make many stops for goalies at the u8-u12 levels. When the shooters get stronger, faster and more accurate, the goalies need to have proper crease movement technique to be in the correct position to make a stop -- and that means skating skills. If they never learned to skate because they were standing in the crease 90% of all u8-12 practices -- they will be unable to learn the proper technique and will be out of the sport when the net time becomes truly competitive. I have seen it happen many times where good parents thought that specializing as a mite or squirt would give the kid a leg up and it only held them back or made them ineligible later in life.
You see the biggest jump in a goalies skill, when a pretty good defensman or a forward makes the jump to playing goalie after 12/13
I feel so bad, I mean really bad, for kids that have been dedicated goalies since mites, and at 11/12 it is so apparent that they cant cut it as a goalie and more importantly realize that they do not want to be on the ice because it is no longer enjoyable
The kids who started playing goalie young to avoid skating around and all the "hard" edge work, can scrape by until about 12, then the lack of skating ability and movement is exposed quickly