How old are the main characters in William Gibson's "Neuromancer"? 

How old are most readers of it?

Well, the target demo according to most TV/Movie/Video game industry
members for Cyberpunk is 18-34.  Moreover, I can tell you that several
universities (including my own, Georgetown) employ it in at least a
handful of English courses which would reinforce that (i.e. it would
encourage 18-22 year old people to read it).

I dont recall any specific references to age in Neuromancer, however
we have several clues: both main characters make references to stages
in their lives that sound like teenage and twenty-something periods,
not to mention in particular the reference to a several-year period of
prostitution for the female lead.