Both players are above 40, but Kramnik is still rated #2 in the world and is rated close to 2800, whereas Anand's rating has steadily been falling. What has Kramnik done to get better with age, which Anand has not been able to do ?
Anand is 5 and a half years older than Kramnik. If you go back 5 and a half years, you'll see that Anand peaked in 2011 with a live rating of 2817.
So the idea that Kramnik somehow kept his rating high in way Anand has not been able to is completely wrong.
The difference between Kramnik's and Anand's ratings is basically just noise. They have been trading places on the rating list for over 20 years. Less than a year ago (September 2015), Anand's rating was higher than Kramnik's by the same amount that Kramnik's is currently higher than Anand's.