It may seem difficult to imagine but just look at it this way: you have a better chance of teaching a person a new principle to live by when they are twelve than when they are 42. Why is that? Because over time a person's decisions shape who they are and it becomes more and more difficult for that person to move to a new path the farther down the rode they are in their current path.
So if a persons spirit continues with them to their next life (assuming a reincarnation scenario) it is conceivable that the it will be more difficult for them to change in that second life than it was for them to change in their first. This is of course assuming that we take with us equally both the good and bad characteristics we gained from the previous life - which would be fair.
And so there would seem to be a law of diminishing returns for these additional lives until perhaps there are no returns at all. Also I consider the fact that we cannot remember what we did wrong (or right) in the previous life / lives and what effect those decisions had on us. This means all we take from the previous life is more of what we became rather than what we knew (intellectually). This also means there is a higher chance we will continue with and repeat the mistakes (and the good) we did in the previous life rather than learning from it. This would slow the learning process down considerably in my opinion.
1. In general, I agree with you that the older someone is, the harder it is to teach them, however, some can and do learn. But where it pertains to one lifetime V the next, that does not work that way. Each life is unique and one does not impact another, save that one evolves over time. Here is a wonderful article written by His Holiness, the Dali Lama, on the topic. http://www.dalailama.com/messages/s...nzin-gyatso-on-the-issue-of-his-reincarnation.