Coming to the present timeline? I think that island they're on has magical properties (potentially speed force related, they were using Judy as a battery), which de-age anyone who arrives there to their teens (Courtney and Emiko were unaffected as they're already teenagers), and suspend their aging - they also don't seem to notice the passage of time. They've been there for decades but don't realise. As such, she's been in the present timeline all along. That's why they can't be sent back to the past, it would actually change history.