If we did pairs of seasons and average them (just a simple average), the last two are the best
I found all the players who had 300+ PAs at every age from 20-35 like Beltre. Then I averaged their OPS+ at each age. Here is the graph with trend line using a polynomial order 2 equation.
Those players were
Beltre averaged a 105.18 OPS+ from age 20-30. From 31-35, it is 138.2. So 138.2/105.18 = 1.314. So he has done 31.4% better. Here are the same ratios for the other guys
I also used wRC+ from Fangraphs. I found all the guys who had 3000+ PAs through age 30 and 1000+ PAs from 31-35. Then I took their Old to Young ratio. Beltre is near the top, doing about 33% better at his old age. The average ratio for the group was .953, meaning they did 4.7% worse. There were 1286 players at the young age and 781 of them made it to the old age. Here is the top 15