"That mixture of overcooked food, bleach, and a whiff of already cleaned-up messes took me right back to early childhood," my boss said of a day care center she visited. "It wasn't good."
Smell. There's something about it that triggers memories on a much deeper level than any visual or verbal cue could.
Smell and early childhood memories
This conversation about day care led me to survey some of my friends about their own experiences with "smell memory", and the results were interesting — everyone had them, but they were all very different. There was the friend who is reminded of her white, Christian, grandmother every time she goes to an Indian restaurant; not because she liked curry, but because some of the spices in her very well-stocked kitchen cabinet are also used in Indian cooking. There was the friend whose proximity to a mixture of motor oil and tobacco induces fond memories of his grandfather, and the one who feels homely and nostalgic when just thinking about the smell of cows. (That one grew up on a farm.)
Indeed, research shows that olfactory — that is, smell — memories tend to come from the first 10 years of your life, while memories triggered by visual and auditory cues relate to events that occurred later on. What's more, smells can trigger memories you didn't even know you had until that whiff reminded you, and smell is more potent, too, in that it tends to evoke a real feeling of "being brought back in time" .
What's more, memory is inherently fallible. That is to say, it isn't unusual for a witness to a crime to report that the car whose driver did a hit and run was red, rather than blue, or to describe a blonde woman when she was really brunette. Not only do people often forget things quickly after the fact, their brains may also add details that were never there. All of this poses significant challenges within the context of justice , but one study showed that olfactory memories are different — they remain the same over time. 
It is also in childhood that we're conditioned to like certain smells but dislike others (though some dislikes are inherent) [5, 6] — the fresh bread your mother baked becomes associated with comfort, a full belly, and love, while you may really grow to hate the window cleaner your cold-hearted, critical, aunt's house always smelled of.
Smell memories beyond early childhood
Though not much research has been conducted into this topic, there is a study that acknowledged that people diagnosed with post-traumatic stress disorder (PTSD) often experience flashbacks in response to olfactory cues. This leads to a few interesting conclusions. First, as the researchers said, that questions about smell memory should be part of the PTSD diagnostic process.  Even more fascinatingly, though, it seems that smell evolved to be part of the evolutionary warning system, as something that's burned into our memory when we experience traumatic events, and encountering which again tells our brains that we're in danger.
Smell you later
The human sense of smell wasn't deemed very important by researchers until more recently — unlike many other animals, we don't rely on it so much to identify other people or things, and we have many fewer genes that regulate olfactory receptors than most other mammals. It does play a crucial social and emotional part in our lives, however.  Could smells "tack themselves" onto events as they happen, thereby acquiring such an important role in childhood memories that our conscious mind has long forgotten? It seems so, and that is why odors from a time long gone bring you right back.