5 year missions are not standard, 5 years is just the maximum time a Constitution-Class starship can operate between major overhauls.
The Enterprise was given a five years mission (most are open ended) because she was already scheduled
to enter orbital dock on a certain date for a extensive "refit."
In the real (original) timeline, it was family friend Admiral Robau who gave little Jimmy Kirk the talk about the birds and the bees. Because George and Winona's attempt had left Jimmy so utterly confused.