Their hearts have not grown old; (22)
This line implies that the speaker's heart has "grown old," but what does that mean? On the one hand it means literally old (as in more years), but it also probably has something to do with the wisdom or knowledge that comes with getting older. Perhaps it has something to do with the way one's feelings (heart) change as we age.
[…] when I awake some day To find they have flown away? (29-30)
We know the speaker is already getting older and, when "some day" comes, he will be even older. The word "awake" suggests, very subtly, that perhaps old age can be a form of waking up, an awareness of new things, the beginning of something new.