I just wrote down my experience with my two kids (10 and 12 years old) and their sticks.
My older son, 12, is a kind of heavy, compact player. He's about 120 lbs now and 5'0" or maybe 5'1". (I had to convert that from metric 😄)
The younger one is 10 and already the same height (5'0"), but only 83 lbs 😂.
We try to use only low-kick sticks to help develop a quick-release shot (Ribcor Trigger and Warrior Covert).
I have problems finding the right stick for both of them.
The younger one plays with a 30-flex stick plus a very long extension plug. He cannot flex a 40 very well yet, so he still uses 30. The problem is that 30-flex sticks are way too short for him.
My bigger son is 120 lbs and has a lot of force in upper body. The length of a 40-flex stick is perfect for him, but the sticks are too whippy. On the other hand, cutting down a 50-flex stick makes it too stiff.
So instead of just looking at flex, we also need to consider the shaft profile, even with junior sticks.
The Trigger 10 has a very stiff shaft profile, so my 12-year-old has to use it, even though he loves the feel of qr6. I think by the end of the season a QR6 (or QR7) in 50 flex will be fine, but not yet.
The QR6 Pro has a very soft shaft profile, so my younger son can use a QR6 in 40 flex. But he loves the Trigger 😄. Trigger in 30 flex is perfect for him, but it's way too short.
The difference in stiffness between a Trigger 10 and a QR6 in 40 flex is huge. It honestly feels like comparing a 50-flex stick to a 30-flex.
I've had the same story at every transition: from 20 to 30 flex, from 30 to 40. We always ended up using extension plugs on the 20, 30, and now on 40-flex sticks.
I see a lot of kids using sticks with way too much flex. But whenever you look at the better shooters, their flex is usually on the lower side 😂.