As people get older, their brand loyalties are more set.
Younger, working folk often don't have the house/family/healthcare/retirement worries of older workers. Marketers
often target young folks because they'll spend on new things.
When millennials share a common experience, such as growing up on AOL Instant Messenger, they're going to share common psychological traits and behavioral patterns.
So, yes, don't be surprised that they behave as a group.
The Millennials are the largest adult demographic: http://www.indexmundi.com/united_states/age_structure.html . Their children will be bigger than them (hopefully!).
People of all ages don't always fit into the "generational" niche the media and marketers try to put them in to.