This par chart shows the rates of using the internet
Prepositional phrases generally consist of a preposition followed by a noun. I would consider revising "of using the internet" so that a noun comes directly after the preposition.
It is clear that the highest number of users was the
young people aged between 16-30 years old with approximately 53% in 1998.
As gmad06 mentioned, the linking verb should be plural. In addition, I would clarify what "approximately 53%" refers to. What do approximately 53% of 16-30 year olds do?
In addition, there was an increase for users aged between 31-51 years old with around 40% while this rate fell to 37% in 2000.
An "increase" of what? As a reader, it is not clear to me what there is an increase of.
To sum up, there was a similar increase for children and older users from years 1999 to 2000.
Again, I would suggest clarifying what there was an "increase" of, although you probably will not need to make this clarification here if you do it earlier.
In contrast with the number of young users decreased significantly among three years.
In contrast with who? I'm not sure what you're trying to say here.