In English we make a distinction - if there are 3 or more persons/items being compared, then one is "the oldest" (or "cutest" or "meanest" or whatever). If there are only two, then one is "the older" (etc.)
The French don't make this distinction. If one is older than any other(s), then she is "the oldest".
Diana, to add to your comment:
A comparative compares a person or thing with another person or thing. A superlative compares a person or thing with the whole group of which that person or thing is a member:
When there are just two members in a group, traditionally, we use the comparative. However, in informal situations people often use the superlative:
Who is younger, Zsuzsanna or Louise ? (traditional usage)
Danny and Hank are both tall, but Hank is the tallest. (more informal)
Because "ancienne" ("ancien") is an adjective, and for an adjective describing a person you need to use "il/elle est". See http://french.about.com/library/weekly/aa032500.htm
Actually, in french we don't make the distinction between something having a recognized sex like a person (or maybe an animal) and something that hasn't a sex. It depends on the genre of the noun. So c' is correct for both it and she. For example: "cette paire de chaussures est ma favorite. C'est aussi la plus ancienne."
I would say neither of those, since listen to a lot of french news/TV shows and I've heard this in use before. Here is an example: La bière est la boisson la plus consommée dans le monde après le thé. C'est aussi la plus ancienne, puisque les sumériens l'ont inventée il y a 10000 ans.
If it can be la plus ancienne, why not also le plus ancien? Marked wrong...
'It is also the more ancient one.' Why would this be a wrong translation, for comparing the age of two pyramids, for example? ..
EDIT: Duo now accepts : 'It is also the more ancient one.' [6 June 2014]..
EDIT 02: Despite a note from Thankwee on behalf of the Duo team that it now accepts 'It is also the more ancient one', this translation was again marked wrong on 19 Jan. 2015. (This time round the word 'more' was crossed out.) Reported again.
You are correct in your understanding of plus. The problem is you didn't consider the presence of la in this example.
une bonne pomme = a good apple
la bonne pomme = the right apple.
What is the difference? Because of all the good apples that are available, one of them is the good one. Not a good one but the good one. The most good of all the good apples. The right/correct apple if you are choosing a good apple.
In this Duo example here, we have the same thing. Not just a more ancient something but the more ancient something. If you are looking to choose from more ancient somethings, this is the one. The most ancient of all the more ancient somethings.