Limitations? Not sure what you mean, but you will need to be
able to engage the students in a group setting. It's harder because
they might be of varying standards.
And as said earlier, do not stop halfway. Responsibility is
important in this field. Be at least there for half a year, or
better, at least one full year, till they finish their end of year
exams.
You can reveal your age; it doesn't matter. I know JC students
who give private tuition to O level students as well.
The most important thing is to be able to motivate your students
to want to score better. You can't do much if they don't want to or
don't have the drive to improve their scores.
Do intro them to Homework Forum and ExamWorld as well. Both are
free of charge too, and should be of some help to most students.
For yourself too, since you are a JC student.
Edit: Just realised this is a thread from Oct -.-"