"When I use, "In men, hair loss starts in the late teens to mid-twenties", I'm saying that the start of hair loss is higher at the late teens than early twenties and mid-twenties? Or I'm saying that the start of hair loss can also occur both in early teens, in early twenties and the mid-twenties?"
Your statement does not suggest greater hair loss at any of the three stages you reference later. It simply says that hair is lost in these time periods; it does not suggest a greater problem in one or the other.
As for in/from, it depends on the subject: tulips start blooming IN the spring, or ON March21, but you will see flowers FROM spring or FROM March 21 UNTIL fall.
Student or Learner