"The anal canal has a triradiate lumen lined by three fibrovascular cushions of submucosal tissue. The cushions are suspended in the canal by a connective tissue framework derived from the internal anal sphincter and longitudinal muscle. Within each cushion is a venous plexus that is fed by arteriovenous communications. These specialised vascular structures allow for enlargement of the cushion to maintain fine continence. In health as in disease the anal cushions appear in the right anterior, right posterior, and left lateral positions.
Fragmentation of the connective tissue supporting the cushions leads to their descent. This occurs with age and the passage of hard stools [or with anal insertions], which produce a shear [pulling] force on the framework. Straining produces an increase in venous pressure and engorgement. The prolapsed cushion has an impaired venous return, which results in dilation of the plexus and venous stasis. Inflammation occurs with erosion of the cushion's epithelium, resulting in bleeding."
British Medical Journal. 2003 October 11; 327(7419): 847-851.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC214027/
"Increasing age was significantly associated with a weakening of anal function."
"Age leads to a consistent reduction in anal function and this is likely to increase the risk of faecal incontinence in old age. From the current data we suggest that in normal women with an uncomplicated obstetric history increasing age is associated with significant changes in anal function whereas long-term effects of vaginal deliveries play a minor role. Moreover our results suggest gradual changes throughout adult life, rather than large changes occurring after menopause."
International Journal of Colorectal Disease. 1997; 12(4): 225-9.https://www.ncbi.nlm.nih.gov/pubmed/9272452
Effects of age on anal function in normal women.