The answer to this question is more than a simple “yes” or “no”. It really depends on what type of autism we are dealing with, so the answer is very situation bound.
There are some types of autism that are so physically debilitating that joining the military or army or police would absolutely be out of the question. People confined to wheelchairs who are highly functioning may qualify for desk jobs, however they would not be fit for combat. Those with Augmented and Alternative Communication devices in use because of their inability to communicate verbally may not be eligible for even these jobs.
One gray area is the question of whether or not those with asperger’s, a milder form of autism, are eligible to join the military. It depends on the extent of their autism and how highly functioning they are. If a person with aspergers is very highly functioning, they may even be fit for combat. If a person with aspergers, however, is so severely debilitated that their daily lives are taken over by the illness, they may not be eligible. For example, a person with aspergers who also has an obsessive compulsive disorder.
Generally, those with autism do not opt to join the military, and are generally not accepted if they do. However, there are these few exceptions.