Now children start using computers at an early age. What’s your opinion about it?

In my opinion, it's never too early late to start to learn something (or start learning something). In this order way(or Therefore), we must start to taking knowledge action as soon as possible, even being a kid I think "at an early age" might be better. The computers are an exceptional tool that the kids must learn to use. In On the other hand  or you can say "However", computer games can take a lot of time and these they can do sedentary kids (could be harmful for kids) if it isn’t well-controlled.
I think that the kids must start using a computer as soon as possible (maybe with at the age of two or three years old) with appropriate content adapted and under supervision of their parents.

Hola Fernando,Good job!

There are still some details that I think you may want to pay attention to:

1.start doing something vs. start to do something

e.g. I start learning English.

      I was learning Spaning,but now I start to learn English. (Here start to do something means that       you stopped doing what you were doing before,and begin with a new task.)

2.sendentary does have the meaning,which you were referring to in your text.However,it is more      often used with "lifestyle" instead of people.

e.g. Obesity and sedentary lifestyles are spreading globally.