J.,
My husband is 50 years old as well (although I am 39, 'only' 11 years difference). When we met, he didn't want to get married at all, but he knew that if he didn't marry me, I'd consider it a partial commitment to me and I'd continue looking around. After living together for a year, he proposed and we have been married almost 10 years now.
There are many things that marriage gives, not least of which is a legal right to joint property without the same amount of headache, health insurance if one of you wants to stay at home, and (in my opinion) a more secure environment for your daughter. You might not think it will come up, but it definitely will at some point... she'll start asking 'why' pretty soon, which actually seems like an excellent question. I think you should really delve into why he doesn't want to marry you (cause if he did he would make it happen).
My husband was divorced and had two kids from that marriage - he told me that he would NEVER put his kids through that again, so until both of us were 100% committed to never leaving and getting a divorce, we didn't have kids. He knew it was important to me, though, and that I was just leaving some of the timing up in the air. Finally after three years of marriage, he was ready and I have a beautiful 3 year old daughter (after some miscarriages). To be honest, though, I'm not sure about having another one strictly because of his age - I'd like him to do 'fun' things with my daughter like bike riding, baseball, etc. and it's tougher for him now to do it. If your guy is 50 and not already a 'vibrant' 50, he's not going to change - I've finally accepted that part about my situation and will either make up for it myself or not have another one. It was my choice to be with him and these are some of the consequences.
Just thought I'd share part of my story - in my opinion, your guy is holding off marrying for some reason even if he won't admit it. Perhaps finding out what that is will help you with your future decisions in the partnership.
Good luck!
S.