The National Customer Rage Study is a cross-sectional study. The organization (Customer Measurement & Consulting) picks a new sample each time. Has the proportion of older people in the US population grown so much as to explain the increase in customer complaints?

Yes.

A lot of what we're seeing is yet again due to the largest generation in US history (the "boomers", so called because they were a population boom) progressing through their lives. Everything from the summer of love (when they discovered hormones) to the OMG we're old! vibecession.