Can an older Christian woman marry a much younger Muslim man?

There are two issues here: the age difference and the different religions.

If you would not see a problem with the older Christian man dating a young Christian woman of the same age, then the age difference should not be a problem.

Some Muslim communities frown on young people dating at all, even within their own religion, preferring arranged marriages. Presumably you do not see this as a problem. Nevertheless patriarchical societies do not like their young women dating men from other societies.

You should be aware that Islam traditionally does not allow Muslim women to marry outside their religion. If you believe that marriage is an eventual possibility, then it may be necessary for one person or the other to change religions, or to have a secular wedding.