Like most things, it is all a matter of perspective. My range is 22 and 13. I think you have it for 20 years, though -- at least based on calendar years.
28 today likely became a teenager in 1987. 16 today will likely stop being a teenager (at least age wise) in 2006. 1987 to 2006, inclusive, is 20 years, two decades, 1/5 of a century -- all of teenagers. People see shrinks for far less.
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