This is the poll they refer to, if you scroll down to the isis question, it's 27% among french 18-24.
The exact French wording they used isn't mentioned sadly. IIRC is was astonishingly bad, as they used the ISIS word, that isn't used by French media, which went from EIIL (Etat Islamique en Irak et au Levant) to Daesh.
It's very likely that most people that answered misidentified the faction that was talked about.
An ICM poll on behalf of the Russian news agency Rossiya Segodnya tested the awareness of the group ISIS (previously known as the Islamic State of Iraq and Levant), in three European nations; Great Britain, France and Germany. The poll revealed that overall 2% and 7% say they are very favourable and somewhat favourable towards ISIS but a greater 26% and 43% are somewhat unfavourable and very unfavourable towards ISIS.
Not only they didn't break it down betwene religions, but I'm not even sure how NYT themselves extracted their "1/4 youths support ISIS" claim from this data.
OTOH, have a look at this link originally posted by imaginenore in some other thread:
http://www.pewresearch.org/files/old-assets/pdf/muslim-ameri...