No, there is no age requirement for IELTS exam but even though, it is referred to give after 14 years.

IELTS, is the most popular International English Language Test, it is designed to assess the language ability of candidates who need to study or work where English is the language of communication.

SAT is basically called Scholastic Aptitude Test then the Scholastic Assessment Test, It was introduced in 1926.

It is an entrance exam which is taken to get admission in Unites States Universities and Colleges for under-graduation. It is a multiple choice, pencil and paper test conducted by college board.

It includes section such as Maths, Critical Reading, and Writing which basically tests how well the SAT assessees analyze and solve problem. It is a general test for verbal and quantitative aptitude accepted for US college admissions. SAT also includes an optional essay section. SAT essay scores are reported separately from over all scores.

YES, It is allowed to use pencil in Writing section because there is an option to choose pen or pencil, but in Reading and Listening section it is compulsory to use pencil.

IELTS Writing Test lasts for 60 minutes, and you will need to complete two writing tasks, each of which requires different text types.

There is no restriction for giving GATE exam, you can give as many times as you want and also there is no age restriction. It is conducted once in a year, first and second week of month February and its scores are valid for 3 years from the date of the GATE result.