Who comes up with the reasoning on ages? There are a lot of us on this forum that are in our early/mid 20's (I'm 25) that have no problem affording these cars. I would expect that anyone who was intelligent, fully employed for a few years, and in a highly technical field or a professional field such as a lawyer or doctor would be ABLE to afford a fairly expensive car. That's not to say that it's a practical or intelligent decision
, but it's not breaking the bank. Of course, there are also plenty of younger people who come from $$ and can afford it that way or a combination of them.
In fact, a younger person who doesn't have extra expenses related to family, living, etc, might even be in a better position to get an expensive car than someone a little older.
But, all that aside, I also agree that the SL is targeted to a more "mature" audience than the SLK. Why else would a car that costs 30-40K more be slower, heavier, less nimble, have more space inside and have more "creature comfort" features...
*Btw, I'm not trying to sound conceited in any way, I'm simply trying to point out that you can't really generalize income based on age. Education and the field you're in also have a lot to do with it.