One thing that has helped me a lot is to socialize with and take lessons from the piano "elite".
Well, that one could be the most tricky. At least it is for me. I guess only things I can do is to wait till December when I'll get extra money at work to pay for three month on Skype.
I started from scratch a week before by 58th birthday. I was lucky in that I found a good teacher locally which was key but I've been consistent in my practising and each week can do a bit more than the week before.I've been having lessons for nearly 3 years now and my aim is mainly to continue to improve so I can play pieces I want but I enjoy playing and it has changed how I listen to music too.I wish I'd started earlier of course but this was the right time for me as I have the flexibility and drive to practice now and I'm hoping to spend a lot more time on my piano studies when I retire.So, in response to the OP I certainly don't think 51 is too old to start (obviously!) and so long as you're enjoying it and feel you're making headway, no matter how slight at times, that's the main thing.