According to this source, radio's streams (yes, including Pandora et. al.) reach 40% of 18-24's, a third of 25-34's, and one out of every four 35-54's.
Interestingly, the results clearly show that while the majority of industry buzz is around streaming video, in all age groups analyzed, TV and Movies (individually and combined) were outstripped by the total reach of streaming Radio / Audio.
While this may seem counterintuitive to some, a possible explanation may be a plus of audio content — it does not require fixed attention or even for the individual to remain in place. Also, Radio and any other kind of streaming can be done on the computer while working on the same device — whether for the purpose of providing background music, sports commentary or other forms of talk-based content.