The number of people reached conceal how old they are (8)
I believe the answer is:
coverage
'the number of people reached' is the definition.
(eg newspaper coverage)
'conceal how old they are' is the wordplay.
'conceal' becomes 'cover' (covering is a kind of concealing).
'how old they are' becomes 'age' (someone's age).
'cover'+'age'='COVERAGE'
(Other definitions for coverage that I've seen before include "news attention" , "Commentary" , "Treatment (of an issue) by the media" , "Treatment of news" , "policy provision" .)