We generally use upper case for constants that are "hard-coded". Or, in other words, when the value is known prior to execution and directly written into the code.

In this code, `birthday` is exactly like that. So we could use the upper case for it.

In contrast, `age` is evaluated in run-time. Today we have one age, a year after we'll have another one. It is constant in a sense that it does not change through the code execution. But it is a bit "less of a constant" than `birthday`, it is calculated, so we should keep the lower case for it.