Frodo,
In just about every society on Earth, this is a HUGE no-no. Girls just beginning to mature into young women are extremely impressionable and vulnerable, and there are always at least some older males that are predatory and will take advantage of these girls. It's been this way through all of time. In response, society has evolved to vigourously protect young girls. It is a criminal offence in most jurisdictions to take this flirtation to the point of "acting on it". Just the flirtation itself will inevitably draw a strong reaction from the adult male "protectors", most likely involving serious violence. It will not end well.
I understand that some chronologically adult people, autistic and otherwise, are socially "on par" with children or teenagers and therefore prefer their company and do interact naturally. Unfortunately, adult males especially can be prone to progress to socially inappropriate behaviour - usually their bodies have adult endocronology even if parts of the brain haven't quite caught up. Consorting with minors isn't exactly encouraged for women, but it is not viewed nearly as unfavourably by society. With the threat of predation virtually nil, these relationships are usually tolerated, cautiously at first, and can be of tremendous benefit to both the woman and the girl.
Unless your friend's protectors (consider that to be every male in her family, and her teachers) personally know you and are TOTALLY CERTAIN that you have no romantic or physical interest in her, expect your relationship to be forcefully resisted. Flirting certainly is not a good idea here.
Society will always put its children's needs above any non-conforming individual's.
Perhaps you should try talking to child-at-heart chronologically adult women instead. They're out there. Lots of them. Around here, you won't even have to look all that far. I'm confident that this will be much healthier for you as well as for the women you meet.
Good luck to you.