The Ultimate Quotable Einstein by Alice Calaprice lists this as "probably not by Einstein". However, this post from quoteinvestigator.com http://quoteinvestigator.com/2014/04/29/common-sense/ traces it to a reasonably plausible source: the second part of a three-part series by Lincoln Barrett (former editor of 'Life' magazine) titled "The Universe and Dr. Einstein" in Harper's Magazine, from May 1948, in which Barrett wrote "But as Einstein has pointed out, common sense is actually nothing more than a deposit of prejudices laid down in the mind prior to the age of eighteen." Since he didn't put the statement in quotes it could be a paraphrase, and "as Einstein has pointed out" makes it unclear whether Einstein said this personally to Barrett or Barrett was recalling a quote of Einstein's he'd seen elsewhere. In any case, the interview was republished in a book of the same title, and Einstein wrote a foreword which praised Barrett's work on the book, so it's likely he read the quote about common sense and at least had no objection to it, whether or not he recalled making the specific comment.
Unsourced variant: Common sense is the collection of prejudices acquired by age eighteen.
Disputed
