Grammar, Usage / Rephrase text about research paper (sentences) [9]

Hello, could you please help me rephrase the bolded parts below?

The area where I want to conduct my research is vulnerable witnesses. The question I will try to answer is: What is the youngest age a child become competent to testify in court?

One of the purposes will be to determine what is the children' age threshold for being heard in court as a witness

Thank you very much