I think there is a mixed bag in both regards. My coach does a panel every year with recent grads from the top 10 schools, not all of them had BFAs and according to them accepted students from their programs ranged from like 21-34 years old. I think the audition itself is the most important aspect, not necessarily previous classes or experience, they want to see where you are and if you will be an asset to them and they will be an asset to you. I think it can't hurt to take class or work with a coach of you are planning on auditioning to help with your pieces, especially if you haven't taken any type of class before. I think knowing 100% that this is what you want to do is also important to these programs and you shouldn't apply unless you know that for sure. They are definitely rigorous (In the US some programs are 4 years for the MFA and they require 6 days a week commitment).
Hope that was somewhat helpful. This is my first year applying, I am 27, I have a B.A. in International Studies and a B.S. in nursing. I have been acting for like 8 years but more seriously the past 3 years, most of my experience is on film and I have trained in Meisner Technique and Method Acting (at The Lee Strasberg Theatre and Film Institute in NYC). I am doing private audition coaching and voice coaching to prepare for auditions in January =D