Which is why I was at pains to stress that it would sound odd in British English Mike. BTW British English is not necessarily the same as English English.
Brit English includes English, Scots, Ulster and Welsh English Although in the case stated it would be the same in all of them.
i.e, "that girl is a sixteen year old" would be slightly odd. It is a structure that would be more fitting to horses, dogs, tortoises etc, rather than human beings. People would know what you meant... but "that girl is sixteen years old" or "she is aged sixteen years "would be more usual. In brit English.
And just because we invented it doesn't mean we don't recognise that it has changed .