Hi Dene,
Does she fits Jane Eyre's role? If your answer is yes, then put all your worries behind and go get your happiness...
I would just like to give you some reflections you should dedicate a more serious thinking, because this is what you and your future wife will deal with once you are married as a couple.
The age gap:
You are 51 and she is twenty, no one can command a heart when you are trully in love with a person but the age really matters, ten years age difference is not the same as twenty. It involves the maturity level of a person, do you remember yourself at the age of twenty? and your maturity level at that age? Does your wife can cope with a more higher maturity level of conversation? Will she feel comfortable if you take her in your group of friends of your age? She will be on her 30's in ten years and you are in your 60's can you imagine you and her after that lenght of time? Put another ten years and she will be 40 and you 70, is she ready to get all through this whatever happens? You will tell me, you are all what she wants and that is your happiness to be together and you are right but what about dealing with this things in our everyday life, to live with this things in our everyday reality? Is she mentally prepared for that? You have already reached a certain maturity and your future wife is just on the beginning of her way knowing and learning what is this world we are living in.
The risk:
Since she is at the Lolita's age, her maturity level is not yet well shaped. You know it by yourself all the risk that you are going through by getting married with someone younger than you and that risk involved your long term happiness.
You have to consider a lot of things because you know that this will not be easy, if you both feel that you are all ready to go through all this come what may it is your own choice and no one can understand yourself more than you. Try to detached yourself from what you feel and seek answers for all your doubts with a calm mind....remember, this is a lifetime happiness your happiness and hers.