Would it be possible to have a disease that only affected people in a certain age group, more specifically, teenagers? And what would the virus be targeting?
In short, you would very likely be targeting specific hormonal changes that only happen around a specific age. The virus would be not really all that age-specific, because the targeted changes aren't; the mortality would be a Gaussian curve centered at around the desired age.
In the real world, the Spanish Flu killed preferentially healthy young people with a strong immune system, because it killed by overstimulating that very immune system.
It would also be possible to have the virus (it almost would have to be engineered on purpose) target sexual activity, acting as a sort of binary poison (in this scenario the disease won't attack older people because they would be already dead - I was thinking of Miri).
Target the latest teen lifestyle trend
Every generation of teenagers has that one trend everyone enjoys and then disappears as suddenly as it appeared. So any of these would be something which would be very specific to one generation of teenagers. Possible disease vectors could be:
- A very popular new recreational drug with a deadly side-effect.
- Some kind of food or drink all the teenagers crave, but which has a dangerous additive.
- Some new fashion trend. For some reason, this generation of teens love to wear scarfs of llama wool. Little do they know that a particularly dangerous and environmental resistant strain of llama flu survives extremely well in fur.
- Some perfume or deodorant with a new synthetic fragrance which isn't actually that healthy. Half the teen population uses and the other half constantly inhales it because someone from their social circle uses it.
- Did you notice that every generation of parents complains that the music kids listen to these days is a bad influence and causes immoral, self-destructive and/or criminal behavior? Well, this generation of parents is actually right for once.
The virus could cause a mutation in cells so they have a higher tropism with somatotropin (human growth hormone, HGH) who's levels are higher during pubescence and teenage year and they tend to stabilize in adults in lower levels.
The growth would be faster in teenagers, causing a cancer like disease, while in adults the growth would be much slower, possibly imperceptible or benign.
The younger the patient, the higher the HGH level and more time for the tumor to develop. If you are 18 years old you could get a milder presentation of the disease and stabilization of the growth after some years, if you survive.
Teens have more active immune systems than older adults; if your disease causes immune systems to overreact and the overreaction is what causes the damage, then teenagers would be much more badly affected than adults.
If you add in that the disease targets growth centers, then you can restrict it further as teens/children grow more than adults. You could also add in a societal distribution method that makes it much more likely for it to be transmitted in/around schools.