My most memory linked smell is elementary school classroom lunches. That warm peanut butter and jelly / spilled milk smell. I went almost 30 years without encountering it and then the moment I did I was back at 6 years old.
That shit is so crazy. I swear sometimes I can sense that specific smell of my favourite dress when I was seven. Or once I walked into a street and it suddenly smelled like the head of a toddler I used to babysit a few years back.
http://www.fifthsense.org.uk/psychology-and-smell/