Create a string formatting function in Python that takes a first name, last name, and age as parameters.
firstName - "John"
lastName - "Doe"
age - 35
def format_name(firstName, lastName, age):
 return f'{lastName}, {firstName} is {age} years old.'

# Usage example
print(format_name('John', 'Doe', 35)) # Output: Doe, John is 35 years old.