I agree with Stefan -- type synonyms are really useful in
developing project by yourself (or a small team), where you are the one
both using and generating the synonyms, and no-one else needs to care.
Then, the simple Age = Int is still very meaningful for yourself in both
documenting and directing the writing.
But in a public
library, finding out that a type X is actually 'Either z' kind of blocks
the way -- you don't know how to operate with X until you become to
know what it really is.. And then you have to memorize this for the
future to stay productive, which is not very easy if there are many
libraries that you're using. (But on the other hand if X equals
something else and more complex, that you don't know, then the synonym I
think becomes more useful again.)