"Do not regret growing older. It is a privilege denied to many."
I agree that one shouldn't regret growing old and changing one's mindset to view aging as a privilege can accomplish this if nothing else works, but it's a cheap low-level wisdom.
It's similar to an employee who justifies staying at a miserable jobs because "I should be grateful I have a job" or an abused spouse who doesn't fight for improvement or leave because "it's worse elsewhere".
There will always be someone less privileges or in worse condition to look to for motivation. Even if you're the most deprived human in the world (if you're reading this you aren't), you can look to history or create fictional alternatives. If anyone can use this trick, then quotes like this aren't saying anything besides, "be grateful at all times".
Let's get to the heart of it then.
The value of your life is intrinsic and shouldn't be derived from the misfortune of others. If you're having to be glad you're not dead to appreciate your life, you're missing something. Life isn't a commodity that increases in value as others fall out unless you're putting it on a market.