It is NOT true that a 'maiden' is always young. 'Maiden' actually means 'virgin', and it has nothing at all to do with age. As a term equivalent to 'girl', the word 'maiden' became obsolete around hundred years ago. You will only meet it in old texts, poems and folk songs dating back several centuries. In relatively modern times, the only time you might meet the word 'maiden' to refer to a person is in the collocation 'maiden aunt' which to refers to the ELDERLY unmarried sister of one of your parents.