The original text can be found on the BBC French language site in an article titled "La pension: toujours en question". And it spells "papy boum". (I am not allowed to post links yet )It is worth noticing that, even in French, it is written "baby-boom" and "papy-boom".
"boum" is the onomatopeia, the French equivalent to.... boom!
"Granny boom" might be possible in English and would actually describe the phenomenon better, since most of the longer living individuals are female.