The problem lies more with the age and standing of the youngest member.
If it’s an underage child, there are problems galore both legal and otherwise. ( And I’m not referring to something like a college Sophomore and a High School Junior where it’s more technicality than significant difference.)
I’m talking about Jerry Sandusky and his ilk, just to be clear.
If both parties are of legal age, then it becomes more nuanced where issues of power come into play. If they are colleagues in the workplace and the younger is clearly subordinate to the older, then there’s a huge potential for exploitation even tho the younger may not see it that way.
It’s really difficult to provide a one size fits all answer without details because there have been many cases of significant age difference without any of the above factors which have worked out fine regardless of others’ negative opinions.
Tony Randall, Larry King, and Don Imus all married women significantly younger for example and they appear to be ok (with the exception of some hiccups in the King situation).