gistfile1.py

class AgeAttr(object):
    def __get__(self, instance, owner):
        return instance.value

    def __set__(self, instance, value):
        if value < 0:
            raise ValueError('Age cannot be negative')
        instance.value = value

class User(object):
    age = AgeAttr()

class Account(object):
    age = AgeAttr()

u = User()
u.age = 1
print u.age

a = Account()
a.age = -4  # ValueError: Age cannot be negative