I am 52 years old. I will be 53 in early May. I never really considered this when I was younger but the answer to this question is literally everything in life can trigger nostalgia in some way or another.
Sounds, smells, tastes, anything and everything can be tied to something in the past. It doesn’t have to be logical. It doesn’t have to make sense. It just happens.
Sometimes you see it coming. Sometimes you don’t. Most times you don’t even acknowledge it, but sometimes it smacks you across the face, punches you in the gut, and kicks you square in the nuts. Other times it gives you a brief warm and fuzzy feeling and then goes away completely.
So consider this a warning, boys and girls. The older you get, the more nostalgia is able to creep up on you and give you a little surprise. Good luck, folks!
Inside the Red Head's Head 
Well said!
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I always found it so poignant and darn poetic that the root of the word is ‘the pain of return’ (algia–‘pain’, like in analgesic, and nostos ‘homecoming’).
I turned into one of those ‘older people’ who thinks that so much about the earlier times of my life was good even though of course I was unhappy then for many reasons, but really, if things weren’t so dismal now, and if I had youth and health and optimism and cinfidence in the world, there might not be such a feeling for me. It’s the sense of chances having gone by, not in a wimpy way of having not dared to do things, but just basics like worlds have gone by–
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