I really don't think waxes cold is a proper use of the word. The lack of heat gets lager. Does a beer stein wax empty? I also don't think the song is all that archaic. Doesn't seem to be. Seems to me the author was just working very hard to "be" poetic, and conversely not trying to actually communicate.

Interestingly enough the version of the song in one of my book by Gavin Ralston is a little less uncomfortable. "Carries colder" is still abit odd but it sings better and is not as ambiguous.

Oh love it is gentle, and love is kind
The sweetest flower, when first is new
But love grows older and carries colder
And fades away like the morning dew

But these are quibbles and are off the point. I was hoping to see some discussion about there being no idea to big for a song.