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2.
Not a child or an adult. Usually somebody over the age of 20 that still displays all the immature characteristics of an adolescent either by choice or not. This sometimes being a positive and negative depending on one's personal beliefs of society and current culture.

Sources:

www.time.com
/time/archive/preview/
0,10987,1018089,00.html

www.sensibleerection.com
/entry.php/44684
d00d yur such a twixter u r 25 and still unemployed!!!1
by ULTRA TWIXTER!!! February 03, 2005
 
1.
A person who is between the age of 18 to 25 and even beyond, who have become a part of a distinct and separate life stage, a strange, transitional never-never land between adolescence and adulthood in which people stall for a few extra years, putting off the iron cage of adult responsibility that constantly threatens to crash down on them.
"Yeah my friend is 26, still lives at home with the 'rents and parties all week, ah the life of a twixter."
by ricbax January 16, 2005
 
3.
a person who is betwixt gender
"Why does mummy have a bulge in his pants?"

"Because Mummy is a twixter darling"
by maddammiss79 July 10, 2008
 
4.
an obsolete term for (a) a boyish girl or (b) a girlish boy
Karl is such a twixter, we sould buy him a dress.
by Dave Hoff May 03, 2005