A:
Hi,
It is always hard to decide which grade to apply to. Professionals always recommend sending students to a lower grade if the family is ok with that.
Benefit of applying to a lower grade is that your child has more time to prepare for college. Usually when students first arrive in American schools, they need a semester or two to adjust to the new environment, and then prove their ability and worth to the school will have another year to prepare for TOEFL and SAT, and then start writing application during the summer after 11th grade ends. Also one academic year gives one more summer vacation and that's a good time for students to build their resume; do some community services, internship, work, etc.
So if your son goes into school as an 11th grader, he won't have enough time to prepare for good universities.
In America, students don't care if one is a year older or younger. American education teaches people to be acceptive of all people. Plus, when students go off to college, age level is very mixed up; students are from late 10s to even 40s and 50s.
For SSAT. Your child is applying to high school, so he will have to take the Upper SSAT.
So study hard and hope you go to the best school that fits your child's interest and dreams.