Those who suffer from autism have a very difficult and challenging life. It is difficult for them to communicate their thoughts and feelings so others can understand them, and often have sensory issues. Even when they are surrounded by a loving and supportive family, they feel like they are alone, which is why so many people with autism suffer from anxiety and depression.
Since it is so difficult for an autistic person to convey their thoughts and feelings, the fact that they are suffering from depression or anxiety is often overlooked. For many fully functional people, expressing the fact that they have either of these conditions can be very difficult, and for someone who already has communication issues it is not surprising that so many people with autism have their depression or anxiety go completely unnoticed.
Identifying an anxiety disorder in an autistic person is extremely difficult, as many who have autism suffer from some level of anxiety on a regular basis. For example, anything that is outside of their normal routine will normally make them anxious. Since people with autism rely on routine to keep them in familiar territory, anything that takes them away from this will cause anxiety, but does not indicate the presence or an anxiety disorder.
Depression and anxiety are common in autism and they can be very difficult to treat. It is very difficult to even diagnose the conditions, and once they are identified they are difficult to address because autistic people have so many communication and social obstacles to overcome.
What usually happens is that a family member becomes concerned about their behaviour, or worries that they may be depressed or anxious and they are then put on medication. While you wouldn't want a perfectly healthy person to be taking anti-depressants or anti-anxiety drugs, with persons who have autism there are some other beneficial aspect of these medications, such as that they improve focus and decrease excessive movement that is common in many autistics.
Generally, as an autistic person gets older they will have more and more trouble functioning in society and may begin to show signs of depression or anxiety (or both). However, with today's medications for these conditions they can soon feel better and will improve even more with some therapy to help them to communicate their emotions.
Parents who are faced with the possibility of their autistic son's or daughter's depression and anxiety may want to consider talking to their child's doctor. He may have a recommendation that will help lessen the despair and hopelessness associated with depression and anxiety so their child can focus on other things.