I would write:
Mr Jones, who will be 45 years old next month, ...
Mr Jones will be 45 years old next month.
Mr Jones will be 45 years old next month; he will then be a 45-year-old man.
The hyphens in the last example prevent the phrase being interpreted as "45 year" + "old man" (unlikely you might say, but to me that's the logic).
In the other examples there is no possibility of misassociation, so there's no need to hyphenate anything.