Why is it so hard to challenge ageism in the workforce?

Despite Australia's retirement age now being 67, workers aged 51 to 55 are often considered to be older workers during hiring processes. The finding comes in new research from the Australian H-R Institute and the Australian Human Rights Commission. But it isn't news for workers in this age group, as several previous studies already found people in their 50s face age discrimination at work. So why is it so difficult to challenge discrimination against old age in the workforce?