I am considering moving to Belgium where the local language is French.
My native language is English and my knowledge of French is, at the
moment, only fair. I do not want to make the move if it then turns out
that I will not be able to learn the language well enough to become
fluent, relaxed and 'at home' speaking, writing and living in French.

I am now 50 years old, though a bright and lively 50. I have seen from
my own experience that languages are much easier to learn when you are
young and increasingly difficult as you get older. I am also concerned
about whether a language learned late in life would disappear as old
age and senility set in.

I would like any information which would help me to assess:

1. how easy it is for older people to learn a foreign language well
enough to become fluent in that language
2. whether people who learn a new language in middle age lose that
newly learned language as they grow old and senility sets in

You can of course assume that I will do my best to learn french and to
immerse myself in the language as fully as possible. In addition to my
two main questions above, I would also be interested (though this is
not the main purpose of this question) in the following supplementary
question

3. whether there is any easy to find guidance on the best way to learn
a foreign language at a relatively advanced age

I would imagine that research has been carried out on these subjects.
I am looking for:

1. a summary of the results of the research in relation to my
questions(if possible)
2. pointers to the research so that I can dig into the details behind
your summary and confirm to myself the correctness of the answer
sufficient for me to brave a life in a new country

Looking forward to receiving your answer