"the elderly often move rightward in the political spectrum as they age"
"kept these elderly in Japan left leaning"
I think both these statements can be true simultaneously. Just because today's elders are more left-learning than today's youth does not mean the former have not shifted rightwards as they aged. In other words, when these elders were younger, they were probably even further left than they are now! (This would not be surprising as their youth would have coincided with the height of the Counterculture era.)
Or to put it the pessimistic way, observing how right-leaning today's youth already are while still young, imagine how much worse they will be by the time they are old..... This is probably what the LDP are betting on happening.