In "The Seafarer," the speaker only talks about aging for a brief ten lines. Nevertheless, it strikes us as a key theme because it represents just how fleeting life on earth can be. Aging becomes a symbol of the transience of earthly glory, of the way everything on this earth fades and passes away with time. The aged man that the poem mentions loses his physical virility and his friends, and is left with pretty much nothing, because you can't take gold, glory, or friends with you into the afterlife. Aging reminds everyone that life on earth is short, but, according to our speaker, life with God lasts forever.
The gold with which the elderly person strews his friend's grave is a creepy kind of substitute for the elderly person himself.
"The Seafarer" uses the idea of old age to show its readers that no matter how much glory you get on earth, when you die, none of that will matter one bit.