Final answer:
Menarche is the first menstrual period and marks the beginning of menstruation. It usually occurs around 12 or 13 years old, but can vary. It's important for girls to understand that menstruation is a natural process.
Step-by-step explanation:
An important point to teach a 14-year-old about menstrual health is the concept of menarche, which is the first menstrual period. Menarche marks the beginning of menstruation and is a normal part of the female reproductive system. It usually occurs around 12 or 13 years of age, but can vary between eight and 16 years old. It's important for girls to understand that menstruation is a natural process and not something to be ashamed or embarrassed about.