Looking at some of the other recent threads has got me thinking.
Looking at my brood the ones that started reception or Y1 in the top group etc tended to stay there going forward.
It's rare in our school for someone to move dramatically 'up' in the scheme of things. Sure a few do but they are the exception rather than the rule.
Increasingly I think that if a child is perceived as being more capable than average early on this becomes a self fulfilling prophecy going forward.
I'd say mine are all broadly similar in intelligence but some have been 'believed' in very early on which has given their confidence and subsequent progress an enormous boost.