1. 40 years of age.
2. between the ages of 40 and 50.
3. a person who is 40 years old or whose age falls between 40 and 50.
[Origin: 1830–40; < L quadrāgénāri(us) consisting of forty (quādrāgén(ī) forty each + -ārius -ary) + -an]
Quadragenarian. Dictionary.com. Dictionary.com Unabridged (v 1.1). Random House, Inc. http://dictionary.reference.com/browse/Quadragenarian (accessed: July 18, 2008).
Wednesday I was 39 but Thursday was my birthday so I am now a quadragenarian; yet no one has taught me the secret handshake!