I had no idea there was such wide variation in the numbers making up her age.

I haven't been here in quite a while, but some time ago I noticed a peculiarity in the version I learned. (From whom? Who knows?! HOW many years ago?)

In that version the last verse is

How old is she, Billy Boy, Billy Boy?
How old is she, charming Billy?
Five times six and four times seven,
Twenty-eight and eleven.
She's a young thing and cannot leave her mother.

(5x6=30) + (4x7=28) = 58
28 + 11 = 39
Both unmarriageable for our Billy Boy, but far from equal.

However, if we change just one word, the operator in the first sub-expression, we can get

Five plus six and four times seven,
Twenty-eight and eleven.

(5 + 6=11) + (4x7=28) = 39
28 + 11 = 39

I have no idea if that was ever in the lyric, and frankly, in this context the word "plus" sounds far too learnèd and technical for the song. But it's a thought.

Mark Mandel
"The Filker With No Nickname"