Well, at the time of their deaths, Helena and Kara were 16. Back that up 9 years, and Helena would have been 7 when Bruce became Batman. But the Batman story has him training for years before that, and also acting as a unnamed vigilante as well before he officially adopts the Batman persona. So Helena was likely trained as soon as she could walk. Kara meanwhile could have been Supergirl for 5 years or so.