Again, not staff, but first question in my head would be "are young visitors representatives of FOSDEM even if it itself is a non-profit?". The language seems to indicate people who represent a non-profit themselves, not those merely visiting an event organised by a non-profit. To me this suggests people like Red Cross volunteers, United Nations interns and many others in similar positions. If somebody is just a 20-year old interested in FLOS software they are probably not "representative of a non-profit organisation".
If they are a contributor to a FLOSS project then the organisation behind the project might help (e.g. the Document Foundation if they contributed to LibreOffice, Mozilla -> quite a few projects, etc.).
Greetings,
Emanuil