These require prompt veterinary attention. When your cat comes in from being outdoors, touch and stroke every part of his body. If he shows sensitivity in any areas, inspect those areas more closely. If you find a small cut or hole in the skin, or some of the hairs on his body stuck or tufted together, there is often a wound underneath. Cleaning the wound and a short course of prophylactic antibiotics can be helpful, although a good number of these wounds will heal on their own with no intervention.
Loose dogs are a threat as well, but are encountered far less frequently than other cats.
And yet despite all these risks the pleasures that cats apparently derive from being able to go outside can lead to a much more contented cat. The real question is why now at 12 years old, do you want to do this. Many people will look to this as an option for getting rid of a behavior problem like urinating outside the litterbox or spraying, scratching the furniture or aggressiveness. Others may do it because someone living with the cat is allergic to the cat or simply doesn't like it. The reasons could be many.