Are you ...
I contain a tin ion
Bones contain some trace measure of tin, and possibly stannous flouride, from some random articles a little Googling found me.
And you cannot find me in a crayon
well, no - crayons have no bones.
But don't worry little peon
You can find me in a swan
You can find me in a heron
You can even find me in a fawn
... all of which have bones.
You need me to move
Without me you will just be like a puddle of goo
Muscles provide movement by pulling against bones (in humans, mammals, etc)., and without a skeleton we would not have the essential structure and shape of our bodies.
You should not let me touch others like me
I promise you, I'm worse than an angry bee
Wearing away of cartilage in joints, such that bones rub against each other, is a cause of great pain (arthritis, back pain, etc.), most notably in older persons as per the hint.
Beware my wrath,
It's worse than an icy cold bath
breaking a bone leaves it first very numb, and then in great pain?
But on the upside - I can help you do math!
Um... a stretch here, but two possibilities come to my mind:
1. you can count on your fingers, which owe their shape to their slender bones, or
2. more likely, dice ("bones") get used frequently in statistics / probability.