They did a good job resemblancewise of the actor. He looked just like Josh, especially the profile. Same nose. It was uncanny.

.
I never really thought about it; I actually thought the actor was a boy.
It's entirely possible that the boy had badgered his parents (mother) into keeping his hair long. In some cultures that's not unusual for boys. In Britain for example, especially in Victorian times, boys had long hair and were even dressed in what appeared to be dresses. Perhaps also his mother was honoring Gittel by letting him have long hair.
Now that you mention it, there is a certain level of dream-like quality and stylization in the flashback sequences many actors who are used in the present-day sequences (Gabby Hoffman playing his mother, etc.), patently impossible details in several scenarios (such as the German sequences), and so on.
All in all, the series is a sort of work of art and not 100% realistic. But I think this is also true of many TV shows.
.