It depends on the scenario. If you only want to compute the age of a person, then use a function since you want to implement a single specific behaviour.
But if you want to create an object, that contains the date of birth of a person (and possibly other data), allows to modify it, then computing the age could be one of many operations related to the person and it would be sensible to use a class instead.
Classes provide a way to merge together some data and related operations. If you have only one operation on the data then using a function and passing the data as argument you will obtain an equivalent behaviour, with less complex code.
Note that a class of the kind:
class A(object):
def __init__(self, ...):
#initialize
def a_single_method(self, ...):
#do stuff
isn't really a class, it is only a (complicated)function. A legitimate class should always have at least two methods(without counting __init__).
