When your child has a disability you eventually learn to accept that development might be slower and behavior might be different than in other kids, but as long as you are making progress then everything is fine. That's why it can be frustrating when a child who has made significant improvement suddenly falls back. There are several reasons that teens with autism might regress in skills or behavior. Often these are a normal part of development rather than a cause for concern.
The National Autistic Society, a British autism charity, says that teens and adults with autism sometimes fall back on old, familiar patterns under stress because the routines are comforting. Repetitive behaviors such as rocking or hand flapping that they seem to have outgrown may resurface as a soothing activity when they are upset. If the stress is temporary then the regressions may disappear automatically when the unnerving situation resolves. However teens go through many other anxiety-inducing situations that may not go away so quickly.
Puberty is a stressful time for any child. Their bodies change shape, they feel new sensations and they deal with experiences ranging from body odor to menstruation. Now imagine how someone who needs predictability might react to that. Although puberty starts before the teenage years for most kids, they are still dealing with changes well into teenage years. Teens with autism might regress to familiar rituals and schedules to restore order to their suddenly chaotic lives. The Autism Advocate blog on the Psychology Today website explains that puberty is hard on children with autism but parents can help them by explaining what is going to happen to their bodies before puberty starts and talking openly about the changes when they happen.
Transition to middle school and high school are changes in environment that can trigger regression into a need for order and repetitive behavior, such as arranging pencils in a specific sequence or tapping out patterns on the desk, that had previously been outgrown. Around these ages are also when many children with autism start to realize they are different than their peers. They hear classmates talk about dating and planning careers, and lower-functioning children with autism may not be doing these same things. The isolation many teens feel as a normal part of growing up can be worse for children dealing with special needs.
Some skill and behavior regression is simply the autism version of teenage rebellion. Teens have a growing sense of self and try to become independent by testing boundaries and flouting rules. Typical kids might skip homework; kids with autism might not practice therapeutic exercises and lose skills. Typical teens might experiment with drugs; teens with autism might stop taking drugs, meaning they stop using the pharmaceuticals that keep some of their symptoms under control.