Hi Eggman.
Something has clearly triggered this behaviour - It is unlikely to be that he has 'come of age' but that something is unsettling him. How does he get on with your older statesman? Are they good buddies? If not, that could be the cause, and you could make it worse by forcing them together.
If they show no issues, then I would look for an external source of the behaviour - Such as another cat, or even dog, coming into the garden / spraying in the area of the doors, etc. Is there a local queen who may be on heat? Has another cat been in the house, perhaps?
Did you have another cat previously, and have recently brought out some of their put away toys / bedding? Any source of new scent should be tracked down for consideration or exclusion.
Once you work out the trigger, then you can tackle it - Getting the plums off is a good start but may not stop the spraying! Tchai, one of my old moggies was de-marbled, and used to stand on the garage roof to spray anyone daft enough to stand below him too long!
A Feliway or Pheromone diffuser may help - You can get them in most large pet stores (for a price!)
Good luck - And get a brolly if he's anything like my old toad!