At a feedback session for a fellow teacher-in-training, the teacher commented that she was a bit nervous about teaching an "advanced" class since she'd been mainly focused on "beginner" classes to which I responded: the only difference between a "beginner" and an "advanced" student is the state of mind from which they're practicing. It has nothing to do with their physical capabilities or how complicated a pose they can get into. If a student is doing what is commonly viewed as "advanced" (based solely on physicality), say an inversion or arm balance pose but the student is ignoring the signals being given by their body and may be pushing too far or trying to show off to fulfill their ego's desire to do the pose, then they are still beginners, despite their physical prowess.
On the other hand, you may have a student standing in Mountain Pose - what is considered a pretty "basic" pose with full awareness of their body and breath, and an ability to maintain this awareness and move in a way that is respectful of their physical limitations, of how they're feeling in that particular moment, and that is solely driven by a desire to simply be present - that is the definition of an advanced student. No standing on your head or complicated arm balances or twisting yourself into a pretzel are required to become an advanced student of yoga.
Of course, I didn't know this at first, and for many years, my asana practice was driven by my insatiable ego. All I wanted to do was conquer more and more demanding poses without really paying attention to my state of mind or how my body might be feeling. Sure, in the moment, I got a real high, when I could finally accomplish a pose like Headstand or Bird of Paradise or Crow, but more than once, I injured myself practicing the very thing meant to care for my body (and mind): yoga hurt me but only because I wasn't paying attention and my ego was in charge. That's what is meant by "beginner" - a state in which we are not yet in control of our mind or desires and are apt to make unwise decisions.
There's nothing inherently wrong with challenging asana poses. I've learned over time though, that the best way to approach them is one step at a time, with the help of a knowledgeable and experienced yoga teacher, and with a constant awareness of my intentions as I practice them (i.e. I'm not just trying to show off but have a genuine interest in the pose for its own sake, and I'm being respectful of my body's capabilities). There are some wonderful benefits derived from these poses, not least being a sense of breaking through fear barriers and surprising ourselves with our newfound strength, flexibility and balance. But these poses should not be the "destination" of this journey, the marker between beginner, intermediate and advanced student because, guess what, enlightenment doesn't instantly happen once you can stand on your head.
Also, as you age, what you're physically capable of doing can change (speaking from experience), and if you're basing a value judgement of your practice solely on what you can physically accomplish, you may be very disappointed when you notice a backsliding as you get older. This may not happen to every practitioner - we all respond to aging differently - but if it does, does that mean you're devolving instead of evolving as a student of yoga? Of course not. This is one of the pitfalls of basing our yoga trajectory on asana practice alone. We would do well to remember that asanas are only one of eight limbs of yoga, and we should also be focusing simultaneously on those other seven limbs if we want to experience all yoga has to offer.
The only thing "advanced" truly means is that the practitioner is able to direct their attention at will and has some control, if not total control, over their mind. That being said, if you're new to yoga, use common sense and don't try an "advanced vinyasa flow" or "advanced power" class as your first one. (You wouldn't attempt a marathon if you've just started running, right?) Just don't be discouraged by the word "advanced". If a class is listed as "advanced", it doesn't mean those students have a better chance at enlightenment than you do. In fact, some of them may be stuck in an endless cycle of ego desire fulfillment and are further behind, spiritually speaking, than someone just starting out. Consider the word "advanced" simply as an indication that a class is more athletically-inclined and it might be advisable to build up your physical strength and flexibility before attempting one. It is in no way, an indicator of your spiritual state. Sometimes, what's labeled as a "gentle" or "beginner" class can do more for our spiritual development than a physically demanding practice, whether we've been practicing for one week or twenty years.
Part of this journey we call yoga is learning to let go of externals: what the ego wants, trying to "sculpt" a perfect body regardless of how that body really feels, and starting to move from a place of wisdom, where we have nothing to prove to anyone else, and our only intention is our highest good.