Hi
My kid used the pack from approx 10-12 yo (not sure of his height back then).
But Pack Fits:
Torso length: 28cm- 46cm (a lot of adjustment is possible)
Waist/Hips: 60cm-91cm
So, it could fit a 9yo depending on their height/torso length and waist/hips.
Some reviews on Moosejaw re age/height
https://www.moosejaw.com/product/deuter-fox-40-backpack_10367529 suggest an age range of 9+ and height of 122cm+
I bought two of these bags for my girls aged 9 and 11. My 9 year old had the bag on the smallest setting and my 11 yr old on a middle setting. My nine year old is pretty short for her age. The bags worked well.
This is the second fox40 backpack we purchased, the first one was for my son who was 7.5 when he got it and this one is for my almost 10 year old daughter. We took these backpacks on several week long backpacking trips and they have been holding up great despite being heavily used and abused. The bottom compartment works great for a compact sleeping bag and one can strap a sleeping pad on the top of the backpack leaving plenty of space in the main compartment. With my kids still being pretty small we were limiting the total weight of the package at a comfortable for them level but the backpacks could hold quite a bit more. I’m sure they will work until their early teenager years. Overall these are great backpacks, good materials, practical and sensible design. The only drawback I see is their high cost.
We bought the Deuter Fox 40 pack for our 7 and 9 year olds. The are both about 4 feet tall [122cm]and we've been on 2 backpacking trips so far. The waistband is just right at the tightest setting so I know there is some room to grow with this pack, which I appreciate. The design is well thought out with all the normal features I've come to expect with a pack and an easily and quickly adjustable harness. I'm 5' tall [152cm]. and was able to adjust it to fit me for a day hike from our back country camp.
The issue finding a pack for my kid, was not the length of the harness but finding a pack with a hipbelt that could be tightened enough to work as a hipbelt, as he was skinny.
That's why I ended up buying this pack from NZ (as it wasn't available in AU) and wanted a pack that fitted properly.
We stuck to Bogong's recommendation of kids carrying 20% of their bodyweight and that worked well, increasing the weight a little bit each year, until he was carrying sleeping bag, clothes, sleeping mat as well as scroggin, water, some food etc.
Cheers