How important is age in CS PhD admissions?
Age is simply not a factor in PhD admissions in computer science, at least in North America.  As Zenon says, age discrimination is illegal here.  

However, being out from school for a significant period of time (say, more than five years) is a potential hurdle for PhD admissions, at least in my department.  We typically recommend that those students pursue a MS degree first, to refresh their academic background.  But since you're in an MS program now, that shouldn't be a issue for you.