Not quite true....so no potential to extend
You can add Audi warranty cover to any Audi for an additional premium, as long as it is less than 100k miles. There is no age criteria.
But check the costs for a 6:
£2376 to extend an existing warranty (12 months 15,000 miles limit)
£2764 to extend an existing warranty (12 months unlimited miles)
£3841 to add a new warranty (12 months unlimited miles)
With those prices, makes it more financially viable to buy the car from a dealer where the warranty is already included in the sticker price.
...if you can find one.