He's sitting in the space port longing to go back. Some current members of the space force kid him for being too old at 19 or 20.
"The Old Man" by Robert Silverberg.
You can see it here, albeit in rather fragmented form. https://archive.org/stream/Imagination_v08n02_1957-04_Gorgon776/Imagination_v08n02_1957-04_Gorgon776_djvu.txt