input: Write a python function to format names with age included.
  assert format_name('John', 'Doe', 35) == 'Doe, John is 35 years old.'
  assert format_name('Jane', 'Smith', 28) == 'Smith, Jane is 28 years old.'
  assert format_name('Alice', 'Johnson', 42) == 'Johnson, Alice is 42 years old.'
output: def format_name(first_name, last_name, age):
      return f"{last_name}, {first_name} is {age} years old."