It's possible that cultural changes brought by longer lifespans will soon begin to play a role too. As parents work for longer, they may be in a stronger financial position to support their older children, as opposed to retiring. But that's not all. Lynda Gratton and Andrew J Scott of London Business School propose that greater longevity will also soon begin to make the "three-stage" life of school, work and retirement feel outdated. And this, they argue in their book The 100-Year Life, may bring particularly big changes for cultural expectations of young people in their early 20s.

"One difference we should consider is the assumption that in our 20s we are meant to go immediately from schooling to a career. In the 100-year life we should consider taking a period of our 20s and dedicating to a new stage, exploration," write Gratton and Scott. "Your decisions early in life impact the entirety of the rest of it… so it is rather absurd that we expect people in their late teens and early 20s to make decisions like what direction they want their lives to take. Instead they should have a period of exploring the world and trying different paths."

What's curious is that this specific period of life, post-teen, doesn't have a commonly known name to describe it, at least in English. Perhaps it should. After all, pre-teens have their own moniker as "tweens". Some researchers have labelled the period pre-25 as "prolonged adolescence", but perhaps another name could be adolthood – spanning the teenage and adult worlds.

And if the idea of adolts doesn't catch on, someone can surely find a better name: after all, from ephebes to younkers to backfisch, we have been coining new categories for young people for most of history.

*Richard Fisher is a senior journalist for BBC Future and tweets @rifish

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