Look chick, I think it is really hard to fit into a catagorie that suits you. I would think that you would be classed as a young mum but other people obviously have different ideas.
I too was a young mum, I had my first DS at 19 and my partner was 17, so we where young parents!!.
I think you would go off how old they currently are, so say you joined a group now, to share the experience of this pregnancy, it would be how old you are now IYKWIM???.
What I have found really hard is that for as long as I can remember I was always the young mum, I had my baby before all my friends and It was a huge shock for all the family. BUT now that I am 26 this last birthday I finally realise that I am not that young mum anymore and I dont fit into that catagory now. It was quite hard to deal with and felt very strange for me. That might sound totally weird but I really dont know how to explain it. I dont think it matters what age, just so long as you are comfortable?