they’re getting on married

[Maybo]

But millennials in the US and the UK aren’t hitting those milestones; instead, they’re getting on married an average of seven years later than their parents, and haven’t married at all. (The tyranny of life milestones by Sara Harrison)

Why do we need "on" in the above sentence?

 

[Rover_KE]

We don't – it's a mistake, and the 'and' which follows should be 'or'.