Why would someone in her late 20s cut herself?

There are definitely better ways to cope. It could be a cry for help. If she is open with it--or actively showing off her scars or cuts--then it is a cry for help. "Look. I am wounded. Can you help me?" And either pride or fear might keep her from saying anything, so this is the only way she has found to express her need for help.

"Attention seeking" has a negative connotation with self-harm. But, it is still clearly an issue if someone is slicing themselves open. It shouldn't be looked down on so much, imo.

If she hides it, then it could be addiction [pain releases endorphins, which can make self-harm addictive].

Alternatively, it could be a way to punish herself for some wrong she feels she committed. Or helplessness, weakness, worthlessness: some mistakes she made.

The reasons are the same as younger people. It's just older people usually have found a better way to cope in that time. Or, they simply never experienced a high degree of pain which required coping mechanisms.

Ahh. Or. If she experiences disassociation, it could be to reaffirm her existence. "I am, because this hurts." Whoch is just a faulty thought process.