Is there a word for the feeling of growing old? Or the feeling of becoming more mature?
Something that describes the way you feel when you wake up in the morning and notice how much has changed:
When the man woke up, he saw his wife, his daughter in the crib, and finally, himself in the mirror, and felt [feeling] as he pondered how fast time had gone by.
It's not really yearning for the past, though, so it's not quite nostalgia. It's a bit more like the way someone would feel when they say: "the days are long but the years are short."