In reply to Heartinthe highlands:
Good question; it depends what you mean.
In terms of the age of the rocks exposed at their summits, I expect A'Mhaighdean must be the oldest, and the youngest has to be either Ben More on Mull or one of the Skye Cuillin summits, as all of these are Tertiary rocks.
You could alternatively look at when the mountains themselves were carved, but it isn't clear at what point they would count as the same mountain they are now; the newest would have formed in the last glacial period, but the oldest could maybe have formed a few glacial periods back. I'm not sure there's really any way to quantify this.
Of course, none of them were "Munros" at all until Sir Hugh drew up his tables. I'm not sure there's any record of how he compiled those; we might imagine that Ben Nevis could have been in there first, and so be the oldest - although that could be very wrong. The newest would, I think, be Beinn a'Chroin, above Balquhidder Glen - as the currently-recognised summit (the West top) was fairly recently promoted and its East top demoted to 'Munro Top' ( http://www.hills-database.co.uk/database_notes.html#beinn_a_chroin ). If you don't like that reclassification, then the youngest would be one of the Munro Tops promoted to Munro Summit in the 1997 revision, I think.