It seems it's not so much the age gap that's worrying as the relationship in general. When my husband and I got together (he's ten years younger than me) his mother wrote me a letter in which she expressed her concerns about the implications of the age age difference in our later years. There was no criticism, just a caring mother's genuine fears for her son. However, she and my father in law made the decision that if we went ahead with marriage they would offer us their support. This they have always done and now, thirty years into my marriage, they and I have the closest relationship imaginable.
It seems as if your son is going to need you to help him pick up the pieces at some time in the future Pigglywiggly, so I'd suggest that you say your piece to both of them about your worries, but then make sure they know that you'll give them all the support you can as the relationship progresses.