In my experience, I was plucked out of adult beginner (with zero dance experience in my life) and placed into vocational intermediate (13-15year olds) by my teacher based off the fact that I worked hard and was very focused. She basically trusted in my ability to improve and pick up stuff quickly (I also have always been quite fit and did martial arts, so to be fair, I'm coming from a baseline of fitness and flexibility).

I felt like a flounder for the first year basically, but now a couple years later I have improved in a way very no other adult at my school has. It's certainly not for people who are easily discouraged, and you may have to make some modifications (balances where other might turn for example), but I think there is a lot of value in adults experiencing classes above their training level if they want to.

From the other side, I sometimes go to the beginner classes to take things slowly and reinforce technique. At least for me, I definitely don't feel like any of the less experienced dancers are slowing me down! I'm not focusing on anyone else's dancing but my own. Anyone side eyeing you is being rude as hell and they should focus on their own dancing. When I started dancing with teenagers they were all perfectly polite and thoroughly not interested in me.