Really, the oldest are mid 30s, and I think even that is a stretch.
Most sources put the end of generation X as 1980. So unless you want to come up with a micro-generation for people born between 1980-1985, we're stuck in the millennial group.
The thing is that if we stick with a 15 year span, the youngest people who should really be called millennials are around 21 or 22, but it seems like each year the lower bound for millennial grows (I occasionally see news reports referring to children as millennials).
I'm assuming this trend will stop when we settle on a name for the next generation.