Maybe a stupid question but how old can a larva be for them to turn it into a queen?
I agree - there's never such a thing as a stupid question. Mind you, sometimes answers can fall into that category, LOL
At around 3 days, the diet given to the larva changes. It used to be thought that larvae destined to become queens received a superior diet, but apparently it's the reverse of this - the soon-to-become queens continue to receive the standard fodder, it's the worker-larvae who receive an impoverished diet, which results in them becoming 'not-quite queens' - but some still manage to develop partially functional ovaries even on that diet, which can cause laying worker problems later-on, if circumstances should provoke this.
So - as Fields says - three(-ish) days is the cut-off point for a reasonable queen. As early as possible for a really good one.
The bees however, will do the best they can with whatever they can lay their hands on - even to the extent of trying to make a queen from a drone larva if their situation should go completely pear-shaped.
LJ