I do understand that the point they're trying to make and I am not arguing for that. Nor am I arguing that vintage cars might be popular - you're going into too much detail (the point was that in our modern world, in regard of products produced, "newer" usually translates to "better").
But what I do believe, is that they're asking the customer to think the analogy behind the idea of not drinking 9 year old milk and connecting that to the idea of not using a 9 yo browser (old = bad, new = good as you said). Wouldn't you agree that for that analogy to work with the message they're trying to convey, you need to make a connection of putting the IE Browser into the milk's place in that analogy?
I mean look at that page: half of your screen is filled with a carton of milk - the other half tells about why you wouldn't use a browser (IE 6) which also is 9 yo.
So wouldn't you agree that somewhere in there lies the notion of IE 6 being as good as 9 yo milk?
Probably?
And remember that most people are bad at remembering numbers. Names, on the other hand, are something that most people do remember better.
Therefore, imho, I don't understand what the marketing department was thinking, as in my opinion, the advert connects your thoughts about 9yo milk to an internet browser (of which we already have created our own views) - and in the long run people will most probably only remember 9 yo milk & Internet Explorer (and bear in mind that this advert comes from microsoft).
Therefore, the question of what was the marketing department thinking rose into my mind. I believe that the point of viewing and "comparing" IE & a carton of milk screams for some serious rebranding for the product called "internet explorer" - because nerds can really relate to that message or what?
And in the end of the day, they are the people who power the revolutions of switching browsers.
tl;dr I don't believe that the advert works in advantage for Microsoft & IE