Age isnt all the issue. He made a choice and you have to decide what your choice will be ftom this date on. Consider the time you have given to this relationship and realize with all of it..he didn't feel it was enough to sustain from being with someone else.
My X was similar except it happened and I asked him to leave..he wound up staying w/ his gf and her mom and her 2 kids (not his) from another relationship. We had 20 yrs also..w/ 5 kids under our belt. Asking any explanations would be redundant because they seem to never know why!