Hi mandimoo:
Our moggies are always on so-called senior food by the time they're 6 months old- at the latest. It won't do them any harm whatsoever.
I don't know if you're old enough to remember the days when we never had any of this rubbish about "animal dietary and nutritional needs". There basically used to be one type of cat food regardless of a cat's age. I don't remember any of ours "suffering" as a result.
In fact it's a bit of a "racket" nowadays to try to con you into believing that you must give your moggies/doggies certain types of grub otherwise it won't get the sort of "nourishment" it needs. Twaddle! It's a money making excercise for manufacturers - and vets whose products cost the earth and are no better than you'll find on the shelf in your local supermarket at half the price.
Make sure you flea the little fella just in case he has "visitors" which he may pass on to your old moggie! We order our Frontline flea stuff online at about half the price than your vet would charge.
There's bound to be the inevitable "power struggle" in the early days between both cats because of course this new "upstart" will try to take over but your old fella will rightly believe that he's king of the castle.
They should find their level in time and though they won't be bosom buddies, they should learn to tolerate each other. Don't be afraid to introduce them under supervision sooner rather than later. Won't hurt.
Make sure also that the whippersnapper has his "crown jewels" removed asap, otherwise mayhem will ensue!
Best of luck.