How old is Mikura?

It's never actually said, but at two points in the series it's strongly implied that she turned tricks. First time, when Mikura explains how the three came together, she said Kenichi found them when Harada was making sex androids and she was...that's where she trailed off and looked at the ground. Second time, late in the series Mikura tells Kenichi she'll have to look for a part-time job, Kenichi replies, "Nothing dirty (ecchi)!"

Flashbacks show that's she's always been tough, ornery and independent, she's lived on the streets without any protection, but that isn't enough to account for her fighting skill. There's no explanation for where she could have gotten formal martial arts or weapon training.