Quoting the question:
“What would you think, having met someone you thought was nice, down to earth..but realised she had OLD self-inflicted scars on her body?”
You state that you thought she is nice and down to earth then put in “but.”
We all go through life and experience emotional pain.
For most of us the scars are not visible but many of us also coped in a self destructive manner. Drinking, drugs, driving recklessly, partying, fighting, promiscuity- all are self destructive but most do not show years later when we have become wiser.
She chose to handle her stress by cutting before learning healthier coping mechanisms.
Does the fact that she coped poorly a long time ago make her less of a down to earth nice person now?
Do not judge her.