Do children need a mobile phone? What is the perfect age for them to have their first mobile phone? Give your opinion. 

     I meet a lot of children who have mobiles phones. You may have seen them too, sitting and looking at them. Many people think that this is alarming.
     A child <em>should have</em> a <em>mobile phone</em>, although it must <em>not be</em> a "smartphone." A smartphone gives them too much freedom, and they do not need that. They need the ability to call their parents, and parents need the ability to call their children; nothing more. Parents should limit this freedom.
     Parents should learn more about mobile phone technology, because many of these devices have parental control options, which have been designed to control their child's use of their mobile phone.

Introductory phrases such as: "I believe that," "In my opinion," "To be honest," and "In summary" should not be used. The reader realizes that the writer is giving their honest opinions, beliefs, and summaries; these phrases add no value to an essay. 
You did not address the "perfect age" requirement.