I'm 32 and technically right on the oldest edge of the "millenial" generation, and have 3 kids (7, 4, and 1); it doesn't necessarily mean "people born in the mid-90s".
If the oldest of the group had to have children under the median, then only those who had children young have them already.
Looking at millenials with children is a huge selection bias:
> Mothers of newborns are older now than their counterparts were two decades ago. In 1990, teens had a higher share of all births (13%) than did women ages 35 and older (9%). In 2008, the reverse was true — 10% of births were to teens, compared with 14% to women ages 35 and older. Each race and ethnic group had a higher share of mothers of newborns in 2008 who are ages 35 and older, and a lower share who are teens, than in 1990.
http://www.pewsocialtrends.org/2010/05/06/the-new-demography...
But I see there is some overlap (Wikipedia):
Milenials : Researchers and commentators use birth years ranging from the early 1980s to the early 2000s.
GenX : Demographers, historians, and commentators use birth dates ranging from the early 1960s to the early 1980s.
Incidentally I have 2 kids.