A couple people have mentioned this already, but this is the way I understand it. If you're less than 7th grade, then you can weigh more than 135 pounds, but you aren't allowed to carry the ball. If you are in 7th grade, you can still play pee-wee, but only if you are less than 135 pounds. It sounds like this is a rule used to allow 7th graders to play if they are too small to get significant playing time on their school team.
Right when I saw this, I didn't understand that, and thought that it was a rule for kids of all ages. I thought that was pretty ridiculous because when I was in 7th grade, I played QB, and I weighed 125, which put me close to the cut-off. And I had a few teammates playing runningback and tight end who I know weighed more than me.
I played in the Chippewa Falls Youth Football League in 5th and 6th grade. Their rule is 100+ can't carry the ball, which is pretty good for 5th-6th graders, in my opinion. Of course, you still get some huge kids playing on the line (I had a teammate who was 222 lbs in 6th grade, I will never forget that), but it's not nearly as big of a deal as an early-bloomer playing RB.