Checking the SSDI (Soc Sec Dth Idx) there are 3 Mildred DeYoungs who obtained their SS cards in MI.One with middle initial of J. and one born after your Mom, eleminates them.But, one looks like a "possible" match:
Mildred DeYoung, b 11 Sep 1891; d Oct 1981; last residence Kalamazoo, Kalamazoo Co, MI; SS card issued in MI.
You might look for an obit for her.Check the Kalamazoo Co, MI GenWeb site, some GenWeb counties are putting their cemeteries online.If not online, post your query, ask if any collection of material or book has been published on local cemeteries, and could someone check for full death date on Mildred for you.Also, some GenWeb counties list volunteers who will do lookups, maybe there is one who does obits.
The closest match for a Robert Yates, who got his SS card in MI, is one who was born 1895, d 1976, last residence is shown as Canada.But, you can check the site for yourself. http://ssdi.genealogy.rootsweb.com/cgi-bin/ssdi.cgihttp://ssdi.genealogy.rootsweb.com/cgi-bin/ssdi.cgi
Note to the far right, "add postem", click and a form will come up, fill it out, let anyone else working on these people, you too are interested in them.