By the way, not all teachers are necessarily brilliant... The argument made here isn't very strong from the start. Your client should go and present it to the company that sold hundreds of millions of iOS devices in a matter of months. You're in North America. There's much more "older" people than "younger" people as it is. Apple sells precisely because it adresses all. Cool has to do with it only in some limited circles - mainly teenagers - and even there, you've yet to define "cool". When daddy buys iPhones for the whole family, he's cool, even though mom and he obviously got one in the process. And if the newest stupid blonde pop singer has got one, well you can be pretty sure the iPhone will remain cool even though the "uncoolest" old fart in the world has got one too.
Movies exhibit iPhones. Celebrities use iPhones. Important businesses value iPhones. Cool cars come equiped with "Made for iPod/Phone" media center and else. The best A/V brands out there are paying royalties for Airplay or "Made..." docks, and they all say they've got the best way to preserve the integrity of your music. Saying that a shift in "coolness perception" - in this case, young people towards older people exhibiting the same device - could unravel such a market is audacious. Pretty sure your client has to think through it all over again. I guess one day Apple will lose some of that "cool" factor we have to admit it has now. But that won't be the end of Apple - it's just that things will have to settle at one point, iOS devices will become, if you may, just another thing. Craziness will find another sweetspot, dust will settle, things will appear as they are, and Apple will still be there, as definite and undisputable as - say - Microsoft. It's not like it's all smoke and keynotes, you know, there's proven, reliable, innovative hardware that has gotten itself in about every corner of the world to consider here.