The sentences need some tightening to make them fit better. There are my edits:
1. Statistics show online purchasing is popular with all internet users, with some variations with those at either end of the age scale (teenagers and people aged over 70).
2. A downward trend can be seen in online game downloads. It is estimated 81% of the teenagers like gaming, while the percentage falls to 25% with people aged between 50 to 70, and a slight rise to 32% with people over 70.
3.Half of all teenagers are also keen on downloading games, with only a quarter of the people in their 30s download games, and the figure drops further to only 6% amongst the seventies group.
On their own, the sentences lack a object (for example, online games and downloads are two distinct items. An online game is downloadable, but so is software), but the general gist of the sentence is alright.