And looking at HM's hair it doesn't seem to be fully grey in that picture so I'm guessing its over 20 years old. Do you know the exact date or at least year that it was taken ?
Though the Princess Royal did get out a very old dress. Her white and yellow coat dress worn in 1981 came out again at one of the Gloucester weddings in recent years, so that was over a 20 year gap in wearings.