Or maybe they realize that their younger self's values and goals are no longer their current values and goals, and give in to a bit of indulgence guilt-free.
It is certainly wise to keep a diary of one's goals, values, reasons and intentions, so that some time later one can tell whether the arguments you made when you are 20 are still persuasive when you are 40. Also reviewing one's diary on a regular basis is likely to mitigate any unintentional value drift.
What is probably not wise is trying to lock your future self in with precommitments. After all, would you want to be constrained at 20 by what your 12-yo self thought? That's how you might think about yourself at 20 when you are 40.
"What is probably not wise is trying to lock your future self in with precommitments."
The outside view suggests that as I get older I will probably get less idealistic and less altruistic. For example, from something I happened to be reading this morning:
...Bob’s lack of ambition, which had initially seemed so noble, began to irritate Jacqui. “Everybody else in the animal-rights movement was growing up, starting to settle down, moving out of the squats and all that,” she recalled. “He was already a lot older, and he was not progressing.” While Bo
When I talk to people about earning to give, it's common to hear worries about "backsliding". Yes, you say you're going to go make a lot of money and donate it, but once you're surrounded by rich coworkers spending heavily on cars, clothes, and nights out, will you follow through? Working at a greedy company in a selfishness-promoting culture you could easily become corrupted and lose initial values and motivation.
First off, this is a totally reasonable concern. People do change, and we are pulled towards thinking like the people around us. I see two main ways of working against this:
One implication of the "won't you drift away" objection, however, is often that if instead of going into earning to give you become an activist then you'll remain true to your values. I'm not so sure about this: many people who are really into activism and radical change in their 20s have become much less ambitious and idealistic by their 30s. You can call it "burning out" or "selling out" but decreasing idealism with age is very common. This doesn't mean people earning to give don't have to worry about losing their motivation—in fact it points the opposite way—but this isn't a danger unique to the "go work at something lucrative" approach. Trying honestly to do the most good possible is far from the default in our society, and wherever you are there's going to be pressure to do the easy thing, the normal thing, and stop putting so much effort into altruism.