My niece has asked me and I told her 'I was hurt a long time ago but the doctors helped make me better.' I don't think it's healthy to lie to children about something like that because once they figure out you lied there is a chance they won't trust you.

As for adults, I hide it at work and pretty much every other adult I'm close to knows and random people don't ask. I'm assuming they don't ask because it's evident I self harmed but I'm not sure.