eMarketer has published a fairly comprehensive statistics relating to online video viewership statistics for the American market. According to the study, this is how the online video viewership market has growin since 2008  (in terms of percentage of Internet users)

2008 : 59.7%
2009 : 63.8%
2010 : 66.7%
2011 : 69.9%
2012 : 73.1%
2013 : 75.2%
2014 : 77.0%

Online Video Viewership By Age
The numbers below are those estimated for 2010 and are provided as a percentage of total internet users in the age group

0 – 11 : 58.1%
12 – 17 : 79.0%
18 – 24 : 86.0%
25 – 34 : 84.1%
35 – 44 : 77.1%
45 – 54 : 58.1%
55 – 64 : 43.8%
65+ : 25.8%

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