> Certainly in an emergency I've seen the bees building queen cells around
> anything from new laid eggs to 4 day old grubs.

I've seen them building on drone larvae, but that is not typical.  You might
like to check out the references I posted recently (just before you wrote)
regarding the ages of larvae selected for emergency queens.

One was

>>>Punnett, E. N. and Winston, M. L. Events following queen removal in colonies
of European-derived honey bee races (Apis mellifera). Insectes Sociaux (1983) 30
(4) 376-383 [En, fr, B]

The timing of queen rearing and swarming are described after queens were removed
from colonies of European-derived honeybee races in Canada. Queen cell
construction began within one day of queen loss, and 91% of the queens which
emerged were eggs at the time of queens loss. Queen cells Were evenly
distributed over the comb face, and 4% of the brood reared as queens were moved
from worker cells to queen cups... (truncated for brevity)<<<

> Anyone got any idea of the oldest grub that will
> produce a functioning queen?

Depends what you call 'functioning'.

allen