Does anyone have an answer?
Will a levain that has been fermenting for say, 18 hours leaven faster than one that has been fermenting for less?
No, not particularly but the flavor will be more fermented and there will be a lower pH in the older one. One might get into problems depending on temperature and the condition of the starter, the amount and type of flour used in the levain.
If a 18 hour levain is not yet mature, then the point is lost. Best to use a starter when it is at it's peak in activity, if continued activity is desired to raise bread dough. That is when the yeast cells are working for you. As peaking activity falls off, so do the numbers of active gas producing yeasts. You want to use a robust starter not a spent one.