My six year old son and nine year old daughter squat and deadlift, so you are probably OK.
Doing sets of twenty is really bad advice. DPH is steering you in the right direction.
We start kids off your age all the time.
The first thing we do is put a loose belt on them and demonstrate how to push their abs out into it. IMO, this is the single most important thing we do on the first day, teach them to use their abs correctly.
We monitor them very closely, until they can show consistent, solid form, and spend a lot of time strengthening the abs, low back, etc, in the meantime. Very light weight is used, with a focus on relatively heavier accesory movements that will assist overall stability in the core lifts.
I have a 165'er that started with us at your age. At first, he couldn't squat 65#'s with good, consistent form, and tweaked his back trying to pull 155 with his buddies one afternoon.
Fast forward 2 years later and he is one of the best squatters and deadlifters I have. His form is what I use to show new lifters what they should look like.