Certainly in an emergency I've seen the bees building queen cells around
anything from new laid eggs to 4 day old grubs.
In an emergency (which over the millenia could be of any source from
volcanoes to beekeepers) the soundest strategy for the bees would be to
get themselves a queen as quickly as possible - if she were to turn out
not very good they could always supercede her at their leisure.
The fastest way to a functioning queen must be to use the oldest grub
which will yield a working queen - maybe this is the reason for starting
with ageing grubs. Anyone got any idea of the oldest grub that will
produce a functioning queen?

Alan Riach - Edinburgh