I am 69 and I do stretching exercises such as touching my toes. Today I discovered that my nine year old granddaughter, who suffers with short achilles tendons, also can not bend over ... barely at all. Would the short achilles tendons affect that? It seems as though her entire body is very stiff.
Generally the hamstrings are the biggest restriction in bending to touch your toes, but essentially anything on the back side of the body can restrict the motion of bending forward. A short achilles tendon, tight gastroc/soleus muscles, tight back extensors etc. can all add to limitation in forward bend.
Stretching: To stretch the gastroc and soleus muscles you need two different stretches - one with the knee bent, and one with the knee straight. The hamstrings cross both the knee joint and the hip joint so these also require two different stretches to affect the lower and the upper parts of the hamstrings.
Sitting: Many kids these days sit a lot more than we used. Tight hamstrings and hip flexors are common with prolonged sitting. Postural exercises can help to balance out the front and back sides of the body. Yoga, swimming, dance are a few examples of activities that may help to loosen up the body in general. An exercise ball in the TV room can help increase activity and reduce static sitting.
Hydration: Proper hydration is also important to keeping a flexible body. It is not just the muscles and joints that need to have their full range. The fascia (basically all the tissue that connects everything) needs to glide. Hydration helps.
These are general things to consider. However, if you think that something is not normal as always check with the doctor.