n. A person who acts younger than his or her age.
downaging pp.

Example Citations:
Experts have coined the term "downagers" — which they say reflects the growing "act younger, live younger" attitude of the over-60s — and believe it applies to more than 100,000 Australians.
—Callie Watson, "Over-60s celebrate 'act-younger, live younger' attitude," The Advertiser, February 1, 2010

The first type of consumer is the Downager, who is Generation Y in attitude but not in age. "You often hear that young people are the early adopters of technology but actually the study shows that the baby boomers spend more on technology than any other generation,' Mr McCrindle said. "They're using the technology, they're connecting, they're social networking and they're texting their kids."
—Rodney Chester, "Reading between the lines," The Courier Mail, February 12, 2010

Earliest Citation:
[This] could answer the needs of Down-Agers, Staying Alivers, and 99 Livers with a Fresh Dessert Sampler for both drug and convenience stores.
—Faith Popcorn, "The Popcorn Report," Doubleday, Aug 1, 1991

This word comes from 1991's The Popcorn Report, where trend-spotter and awkward-neologism-generator Faith Popcorn coined the phrase down-aging (or, actually, Down-Aging, since Trends Must Be Initial-Capped):

This refusal to be bound by traditional age limitations is the trend we are calling Down-Aging: redefining down what appropriate age-behavior is for your age.
—Faith Popcorn, "The Popcorn Report," Doubleday, Aug 1, 1991

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