Let G be a finte group and a ∈ G.

since <a> is a subgroup of G , so the order of the subgroup must divides the order of G.

that is,  |<a>| | |G|

here, |<a>| = |a| = o(a)   : the order of the element a.

therefore,
the order of every element divides the order of the group.