I fear I'm now too elderly to know exactly what a millenial is, but I'm going to take a stab that it's somebody born this side of 2001 (or 2000, but let's not get into that).
If this theory is true, the cereal-shunners can be no more than 15 (or 16) and should jolly well eat what they're given and be grateful! [Cue dire warnings about starving children in Africa, etc.]
ETA: OK, apparently they're people born before 2000. In which case you can't tell them anything. Carry on.