For sentence (1) it would be correct to say: You can see how many people in Australia use online services, based on age group and purpose; OR: You can see what percentage (or percentages) of people in Australia use online services, based on age group and purpose.
You can therefore use percentage in the singular here,to refer to the use of online services, or you can use it in the plural, to refer to two things, namely age group and purpose and the percentages associated with each age group and reason for using the internet.
As with many language questions, there is seldom only one correct answer.
With regard to question (2), the meaning of neither sentence is clear, in other words I can't work out what you are trying to say. Perhaps you could provide the actual example and some context? I apologise that I am not able to assist you with your second question.