From which level of education, is it better to choose teacher assistants?
Some universities have rules about what kind of student may be a TA.  Assuming no such rules exist, the TA should be qualified to fill that role, whatever that means for a given course.  That is, the TA should have both the domain knowledge and pedagogical ability that the job requires.

It doesn't matter what level of education the TA is at. In my own department it is quite common for TAs to be "peers" of the students in the class (B.S. students hired to TA the B.S. level courses, M.S. students and PhD students hired to TA graduate courses including both M.S. and PhD students.)