What a shame. Personally, I've been hooked to motor-bicycles since around early last year (I'm 16 turning 17 in a week) and meticulously read over every FAQ and tips thread on this forum before getting an HT - but I can see why some other teens might not fit this hobby well.
For sure, if I wasn't a gearhead I wouldn't see much of the value of the motorbicycle; until you actually look up what they are, do you not realize that they are actually very sensitive and personal machines that require constant care.
However! I don't think it's a matter of values and "taking care of things". I think it's just that teenagers these days are commonly taught that most things are nearly maintenance-free and anything broken has a designated type of person to fix it - for example, in the 60's one might have worked to adjust the valves on a Beetle every few thousand miles in their own garage but now so many things are maintenance-free that teenagers are not taught to take care of their own stuff. This behaviour is, obviously, compounded by the fact that the things a lot of teens are interested in - things like cell phones and video games - are constantly improved and as such will usually be due for a replacement by the time any maintenance is required. Therefore, it's not that teenagers necessarily are simply less respectful these days; they are taught that most things don't really require care and they should focus on what's important (studying and social contact and such) rather than fixing what doesn't need to be fixed.
In conclusion, with everything getting more and more complex, "do-it-yourself" is generally dying out in favor of simply calling a repairman. The HT engine, for example, is contradictory to the things that teens are exposed to these days: it's very finicky, it's very unreliable, and it's very sensitive to everything. So, it's not really that surprising that the hobby is more popular with the older generation who grew up fixing everything themselves and this generation of teenagers doesn't take well to this kind of thing.
With the evolution of new battery technology, inherent reliability, easy installation and rising gas prices, however, we might see a general increase in E-bikes instead.