What is the order of the Identity point on prime order elliptic curve groups?
For something to be a group, it must have an identity. This is a definition.
The order of an element, $g$, in a group is defined to be the smallest positive number $n$ such that $g^n = 1$, where $1$ is the identity of the group.
By this definition, the order of the identity in any group is $1$.