Those in employment may become a member of the Health Insurance Society. This is known as becoming an"insured person", and it applies to everyone automatically.
When you become an insured person, you have the right to receive insurance benefits and are obliged to pay insurance premiums.
You qualify as an insured person the day you are employed, and are disqualified the day after you retire or die.
Once you reach the age of 75, even if you remain employed, you will no longer be considered an insured persons under Health Insurance Society coverage. Instead, you will be insured under the medical service system for persons in the latter stage of old age.

See here for more information: Persons aged 75 or over to be enrolled in medical service system for the persons in the latter stage of old age.

Health insurance coverage for those working reduced hours (e.g., part-time employees)

Those whose designated number of working hours per week and designated number of working days per month are at least three-fourths those of full-time employees are eligible for health insurance coverage. However, even those whose numbers of working hours and working days are less than three-fourths those of full-time employees are eligible for health insurance coverage if they meet all five of the following conditions:

1. Their designated number of working hours per week is at least 20 hours.
2. They expect to be employed continually for at least one year.
3. Their monthly wages are at least 88,000 yen.
4. They are not students.
5. They work for companies that consistently employ more than 100 insured persons (specially eligible establishments) or ones that employ 100 or less but where labor and management have agreed to join health insurance.