A number appearing at the beginning of a sentence is written out:

Sixty-five years ago this was open rangeland.

Fourteen inches of insulation provided a thermal barrier.

EXCEPTION: If the number is very large, writing it out would make the number difficult to read quickly. In these situations, the sentence is rewritten:

not this

765 students responded to the request for volunteers.

nor this

Seven hundred sixty-five students responded to the

request for volunteers.

 but this

The request for volunteers elicited responses from

765 students.