I am mother and father,
but never birth or nurse.
I’m rarely still, but I never wander.
What am I?

What walks on four legs in its initial years,
two in its mid-years,
and three in its later years?

Pronounced as one letter,
And written with three,
Two letters there are,
And two only in me.
I’m double, I’m single,
I’m black, blue, and gray,
I’m read from both ends,
And the same either way.
What am I?