As Paramanand says aptly in the comments :
The fact that you get high quality answers is more important than the information that the answerer is/is not a professional mathematician.
First, I will answer the question : By my estimation, I have been on the site for about three and a half years. I would like to mention that there are quite a few experienced and retired professors on this site. Without taking names, the one I know the best is a retired professor who loves probability from the bottom of his heart (I've had the pleasure of seeing him in person). He is a very frequent poster here, in the field of probability. There are some that are all rounders, and I must admit that having an interaction with any of them is an enlightening experience.
Some very famous people rear their heads often on this site. Did you know Noam Elkies is on this site? (If you don't know who Noam Elkies is, then find out : I had the pleasure of a chat with him on this site).
A lot of people on this site are actually probably just out of, or are in their PhD stage. What helps at that stage is that they are real all rounders in terms of their knowledge, but also have a shade more experience in some specific area of mathematics. In my case , that would be Markov decision processes and PDE.
Therefore, most answers you will receive to non-elementary questions will be from these people, or from people with more experience. My last seven or eight questions on this site have all been attended to or been answered by people who I know are in their PhD stage or are professors.
Ideally, posting a question of high quality, or with a specialized tag, gets the attention of the resident expert on that subject. For example, putting up Stochastic processes will ring somebody's doorbell(somebody who's a real expert by my admission).
Other than that, we also have people who are inexperienced by their own admission, but you know what : there's a difference between knowing something, and communicating that to someone else. That's why the best professors are not necessarily the best teachers (the best footballers are not necessarily the best football coaches, as an analogy?). The best teacher I know of a subject is not even on this site : she taught my neighbor origami, and she is now twelve years old!
So remember, there are a lot of experts on this site, and I am sure we are all friendly and can have a chat with you on whatever you desire : but the quality of the answer is all that matters in the end, and I can confirm to you that a non-professional is also very capable of giving excellent answers on elementary questions : occasionally better than experts , I opine.