The argument or discussion isn't on what people can and afford and at what age. It's whether or not age devalues a brand.
Personally, I don't agree with what that post. That poster seems to have surveys that prove that they do?
If a person had money to buy a loaded E-Class, why would that person even care that a 20 year-old drives a CLA? Or why would that person be turned away? If you're buying into a BMW, Audi or Mercedes, either you're an enthusiast or you're into it for the badge. If you're in for the latter, what other brand would you go for to show off your wealth that offers the same level of practicality? There isn't, it's just the German 3 here in North America. You think that person would go to a Lexus? No, because if they were only in it for the badge, in that person's mind, there's nobody outside of BMW, Audi and Mercedes.
Cars like the 7-Series, S-Class and A8 make the brand be seen as premium marques and makes the lower models aspirational like the 1-Series, A3, CLA. But those "lower class" cars also do the same to those higher models. Majority of buyers will only be able to afford a C-Class, 3-Series or A4 but being able to afford a 7-Series, S-Class or A8 makes the consumer feel like they've made it in life. The only way this theory doesn't work is if those lower models were garbage.
So again, I don't see how that theory works even if there are surveys done. It's even harder to prove because North Americans only see the German 3 as the only true premium marques even if there is Lexus, Infiniti and Cadillac out there. The truth is, if you were to ask anybody what premium car they would want to own, they're going to name a German brand.
If a person is going to buy a 7-Series, S-Class or A8, I highly doubt that person really cares or even thinks about teenagers driving a 1-Series, CLA or any other lower models.