If depressed by your age
Remember the sage
Who by numeric twist
Would thereby insist
That at year Twenty-one
The age of great fun
A mathematical quirk
Commences to work
Stopping further addition
From then to perdition.
He said
“Double your age,
Then add an ‘0’
Divide by your age,
C’mon,don’t be slow!
Now add a year
There’s nothing to fear
‘Cos now you will see
That you, too, must be
Just Twenty-one,
Perennially!
George Potter (after Edward Lear)
George's Rhymes for the Times 