Yeah, we know "So We'll Go No More A-Roving" sounds a lot like a kid's song, but that doesn't mean it can't be about death. If a sword outwears its sheath, that's pretty much it for the sheath right? And if that wasn't clear enough, the speaker talks in the next line about the soul wearing out the body—in some religions, when the body dies, the soul leaves. So, it is about death. Now how does roving fit into all this? Well, the speaker wants to stop roving because he can sense that he's getting older, that death will eventually come, and he doesn't want to be roving around like a crazy frat guy when he should be doing more serious things leading up to the end of his life.
You don't have to be old to be worried that death is near. It's uncommon, but totally natural. Just witness Byron.
The approach of death can make us realize that we've been leading a purposeless, unfulfilling, and roving life. Gee, thanks Byron.