The answer to that is...well, long, honestly, and necessitating a little research, which is why it hasn't happened yet.

From what I've seen, it does change for boys at the time of their bar mitzvah; for girls it seems to be ridiculously young? Mind you, that was from a quick glance at Wikipedia, of all places. I'd like to go looking on more specialized websites before stating that that is in fact the way the law was drawn up. I'd also like to find out how it's handled among people who actually follow it today, though even there I expect to find variation--ever heard the expression "two Jews, three opinions"? Yeah, that often holds true, particularly in matters of religious law. That's probably one main reason why we don't have an equivalent of the Pope--while there are rabbinical councils and chief rabbis and the like, they're often localized in some way. There is definitely no one rabbi who can speak for (or make laws for) every practicing Jew in the world. I'm not even sure the newer streams of Judaism (Reform, Conservative, Reconstructionist, etc--as far as I know, Reform is the oldest, having started in the 1800s) listen to one main association of rabbis. Could be wrong about that, though.

At any rate: more research needed, but for boys, at least, I think the bar mitzvah is the point of change. I also wouldn't be surprised if a version or two mirrored it for girls nowadays. Bar/Bat Mitzvah is definitely a turning point--officially, you're an adult at that point. What that means in practice in today's world, where thirteen isn't considered particularly old, is that now you're responsible for observing certain mitzvot (commandments)--fasting on fast days if you have no health problems preventing it, lighting holiday candles (generally for girls), wearing tefillin during prayers (generally for boys), etc. I was given my own set of candlesticks for my bat mitzvah, for example; before that, I think I'd often stand with my mom when she lit hers, maybe even sometimes light one of them? What I'm more sure of is that if my mom was away on Friday night (infrequent, but sometimes happened), my dad and I would often each light one candle. And I'll stop before I go off on a tangent of how many candles people light and why (short version: some traditions have you lighting one for each child, which can make for a lot of candles and is very pretty).

So...yeah, I guess that's my response to that particular question for the moment. Will add if/once I get around to doing more research, since I haven't run into this aspect of it personally too much--not enough to say anything beyond "yeah, I started wondering once I had slightly less small male cousins, but never quite asked", anyway.

~Z