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.

1. Grammatically acceptable but scientifically illogical. It cannot be equal and less at the same time. Have you defined teenagers in an earlier paragraph? 2. Depending on your methodology, estimated is not a good word as it implies you guessed at a result. 2. And 3. refer to two different groups. One is over seventy and the other 70-79 years of age (excludes 80 and above). You have to be consistent with your groups.