Between twelve and twenty people? (9)
I believe the answer is:
teenagers
'between twelve and twenty people?' is the definition.
(people aged between twelve and twenty)
This is all the clue.
(Other definitions for teenagers that I've seen before include "Young people" , "They're between the onset of puberty and maturity, generally" , "Adolescents" , "Youngsters" , "They're aged between 13 and 19" .)