Important videos/data should be backed up to more than one media: multiple hard drives + optical discs, even tape drives. Accelerated aging tests are useful up to a point, but only offer a prediction. The real world is very different, and as tomwil said you're more likely to have trouble finding a good player/drive 20 years from now than anything else. The best strategy is to make duplicate archives to new formats every few years.
Gold discs cost a fortune because of the gold content, which allegedly resists decomposition better than the cheap reflective layer in standard discs. But the gold layer is not as reflective as standard aluminum, causing reading problems with some drives. So the alleged durability advantage is cancelled out by the significant possibility of reading errors years in the future. And gold dvds aren't made any better than "normal" dvds in other respects: they're equally susceptible to dye layer decomposition (esp if air gets into any hairline cracks that develop between the two plastic halves of the disc).
If you specifically need video DVDs, the very stable AZO dyes used in premium Verbatim discs offer 90% of the theoretical durability promoted by gold. If you don't particularly require a video DVD compatible with (rapidly-obsoleting) hardware DVD players, you might consider going with Blu Ray instead for long term video and high-capacity data storage. Standard BD-R technology is extremely similar to the overhyped M-disc, but less expensive per GB. For all practical purposes, you could say M-disc is simply BD-R tech retro-fitted to the smaller-capacity blank DVD format. I don't see the point in using such a Frankenstein non-standard drive/blank DVD system when you could easily choose the standard, much higher capacity BD-R. The only "gotcha" with BD-R is being careful not to buy the less-durable alternative "LTH" media, which is dye-based like recordable CD / DVD and has the same drawbacks. Standard "HTL" BD-R is the version akin to M-disc.
Some of us have been in this game long enough to experience our own "aging tests" - and have been surprised by some of the results. I have a huge number of various no-name CD-R discs made 14 years ago that are still perfectly fine, and quite a few ten-year-old garbage-brand dual-layer DVD-Rs that still play great (despite expectations they would rot within the first month). Even a few La Cie USB portable hard drives I bought in 2001 still read perfectly. OTOH, I've experienced read failures with some expensive high-end media in much shorter amounts of time. Ya never know, none of us knows.
