[Air-L] Internet statistics

I need some help with Internet statistics. Every source I see show the percentage of each group that uses the Internet (e.g. 98% of those 18-29). I am looking for statistics that indicate of the total Internet population, what percentage use the Internet (e.g. of the Internet population, what percentage is 18-29 so that, in this case, age figures would add up to 100%). I wonder because we always compare our samples to Census figures when the proper comparison should be the Internet population.