What does it mean, to be alive? Beyond the most trivial physiological sense, I am not at all sure there is a genuinely shared meaning for this expression. That is to say, when one person says it (or writes it), the meaning they had in mind is very likely different from the meaning actually conveyed to the listener (or reader); and to have at least some meaning for this expression, one has to have experienced at least two different mind/body states (one: being alive, and the other: not quite alive); or maybe even a whole inner scale of being alive.
I don't know about you, but I still remember the time when I didn't have this scale within me; no inner distinctions between being and not being alive. I guessed people mean something by it (apart from being simply not dead), but no appropriate experiences, no consciously perceived differences in sensations.
Which brings me to a thought I've been entertaining for some time. There is an apparent sense in which younger people are more full of life and spirit; more interesting; more promising. As a person ages, it's happens all too often that they set into an "auto-pilot" routine, just going through socially determined moves of being a "responsible grown-up"; all in all, growing less and less alive. But on the other hand, there are some essential qualities one can only acquire through years of experience, tragedies and joys, failures and successes; so those people who are fully alive in spite of being older, are (in a sense) more fully alive and human than their younger selves.