"As expected, adults with ASD [autism spectrum disorder] experienced more psychological symptoms and distress compared to a typically developing comparison group." But: "lifetime diagnoses for any psychiatric disorder were less often present in older than in younger adults with ASD, suggesting reduced psychopathology in late adulthood, a pattern that has been commonly observed in large typical aging studies."
Those were some of the findings reported by Anne Lever & Hilde Geurts  (open-access available here) based on the examination of "psychiatric symptoms and disorders in young, middle-aged, and older adults with and without ASD" (N=344). In a two-stage study, researchers first delivered a questionnaire about psychological symptoms and medication usage including "the AQ [Autism-Spectrum Quotient] and SCL-90 [Symptom Checklist-90 Revised], among other questionnaires." Stage 2 consisted of the "administration of a neuropsychiatric interview to examine psychiatric disorders and an analysis of potential risk factors" including use of the ADOS (although probably not the 'boiled down' version).
After quite a few bits of statistical work, a few potentially important trends are reported. As per that opening sentence, the ASD group as a whole showed higher scores on the SCL-90 compared to controls. The figures for lifetime frequency rates of various DSM-IV disorders were also generally higher in the ASD group - "79 % met criteria for a psychiatric disorder at least once in their lives." In line with other independent peer-reviewed research, mood disorders were the most common issue detected (57%) across all age ranges in the ASD group, followed by depression (53%) and anxiety disorders (53%). Attentional and behavioural disorders were present in about 30% of the cohort and the increasing interest in obsessive-compulsive disorder (OCD) with autism in mind is also mirrored in the Lever/Geurts findings. Also: "Over 65 % of the adults with ASD meeting criteria for any lifetime mood or anxiety disorder, also met criteria for the other co-occurring disorder." Birds of a feather and all that...
A few other details are also worth mentioning. The severity of autism (as judged by both self-report and ADOS) seemed to have an important bearing on the "amount" of depression and anxiety symptoms measured. Being female was also a strong predictor of the occurrence of mood disorder in ASD too. Finally and to repeat myself: "lifetime diagnoses for any psychiatric disorder were less often present in older than in younger adults with ASD, suggesting reduced psychopathology in late adulthood."
I don't really need to say much more about the findings of depression (in its various forms) being over-represented in cases of autism (see here) nor that anxiety disorders might also figure (see here). The peer-reviewed data in those areas speak for themselves. Attentional disorders, as most prominently manifesting as attention-deficit hyperactivity disorder (ADHD) is also not necessarily a new comorbidity to autism research (see here). The idea that age however, might be something of moderating variable on the expression of these important comorbidities is an important point and taps into a growing realisation that science needs to look further at the long-term manifestation(s) and prospects of autism into adulthood and beyond (see here). At the moment, I don't have any really good ideas as to why comorbidity (reaching clinically relevant thresholds) might be less likely in older adults with autism than younger ones. I have to say that I don't really buy into the psycho-babble explanations that might be put forward; perhaps being more inclined to suspect biological (or gene expression) changes accompanying the ageing process as potentially being more pertinent. I might also suggest that the autisms (yes, plural) of yester-year might not necessarily be the same as the autisms of more recent times, which could also exert an effect. More longitudinal research would perhaps test this hypothesis.
Irrespective of the reasons, the headline conclusion that clinically relevant psychiatric and behavioural (and perhaps non-threshold) comorbid symptoms are over-represented in cases of autism places the emphasis on appropriate screening and intervention where possible. Once again, autism does not generally appear in a diagnostic vacuum (see here) and that also goes for somatic comorbidity too (see here). That somatic and non-somatic comorbidity might potentially be 'linked' (see here) is an additional point to make...
 Lever AG. & Geurts HM. Psychiatric Co-occurring Symptoms and Disorders in Young, Middle-Aged, and Older Adults with Autism Spectrum Disorder. J Autism Dev Disord. 2016 Feb 9.
Lever AG, & Geurts HM (2016). Psychiatric Co-occurring Symptoms and Disorders in Young, Middle-Aged, and Older Adults with Autism Spectrum Disorder. Journal of autism and developmental disorders PMID: 26861713