Definitely depends on the performance and the kid. DS has always been amazing at sitting still. Incredible, since when he is *not* sitting still, he is bouncing all over the place. In any case, we started with free afternoon concerts around 18 months. I think his first at night, pricey tickets, not kid event was around 3. He's also been to several "house concerts" where he is invariably the only child. At 6, I would trust him to behave well at almost any venue with almost any performance. I'm not sure he would enjoy an opera, but I would absolutely trust him to get through it.
Still, he only gets to go to our favorite musical theater place once a season, because that is our couple time. The theater has been amazing about explaining what their age limits mean when we pick the show for him. For example, a show with a lot of dancing and action is great for kids. Pippin has that, but isn't so great because of some of the themes. They advised us not to take him to Camelot because even though there are plenty of knights, it's basically a romance, which is boring to six year olds.
Our daughter, at almost 3, has been to almost nothing. She is more committed to her nap schedule and less committed to sitting still.