What happens to human dopaminergic reward system once a teenager becomes adult?
This may shed light on why the teenagers response to dopamine changes. Quote from reuniting website


  There's much still to learn, but it looks like a number of reward
  circuitry events occur after climax that have the potential to
  desensitize us for a time. First, androgen receptors decline after
  ejaculation, and take up to seven days to normalize. (That means the
  effects of testosterone on the reward circuitry are probably blunted
  for a while, quite possibly affecting outlook.) In addition, opioids
  released during copulation hang around for a while, apparently causing
  lingering declines in oxytocin, which hamper sexual responsiveness. As
  noted above, there is also likely a drop in responsiveness to a
  neurochemical vital to our sense of well-being: dopamine. In effect,
  the brain has changed. It now requires more stimulation to get the
  same pleasure response as before, and sometimes no amount of
  stimulation will truly satisfy—until it recovers. Anecdotal evidence
  suggests that such changes affect both sexes.


The author further suggests that orgasm and ejaculation increase prolactin levels for up to 14 days, during which the brain is less responsive to pleasure and requires novel entertainment or partners to produce pleasure.

From the info on that site, it seems that the 14 day dopamine-affecting cycle following orgasm continues for as long as a person is sexually active (with orgasms). With this in mind, it appears that the adult response to dopamine release is the same as teenagers who are sexually active.