Thank you for your message. We show the comparative and superlative forms of the adjective at the top of the page, but I appreciate that doesn't help you translate them. This is really more of a grammar question than a vocabulary one though, as it's a question of knowing how the superlative is formed in French. As @DearPrudence suggested, I've added an entry for '-est'. I've also made sure that 'oldest' appears in one of the example sentences for old, so that hopefully you will at least be able to see its translation in an example.
That change to the sentence will have to go to our translation teams for review, so you won't see it in the dictionary until that's been done and the dictionary's been updated. The new entry for -est will be included in future translation projects.