Use classification when the number of categories are limited and nothing in between makes sense. For example a class is either a dog or a cat nothing in between.
But when it comes to something like ratings, 3 is as likely acceptable as 3.5 ((so is 3.56, etc) so you are not bound to only one value among others. it can be in between as well.
Apart from this,
age is kind of interesting as some people may see it as a closed number of classes(classification), while others treat it as a regression problem, as the individual can have any age and its not fixed to an upper bound in theory. A person is likely to be 1 years old, or 120 years old! it could be something in between as well.
Then again some may say that, we know people normally don't live past a certain age, like 120! so we create 120 classes, and people can be in any of such categories and it makes sense and is enough!
Just note that this may not apply to all situations (and it does/will not), when you for example are going to lets say, get the age of some substances, this may not hold as easily. also it does not apply when you have unbounded ranges.
age is inherently a regression problem, that based on the situation may be represented as a classification problem by some.