for Christians: when does the individual 'Fall into sin'?

[Mr. Smith]

Option 3

That's why the LDS Church doesn't baptize babies, for they are still innocent. Why we wait until 8...I guess you could call it an inverted communion.

Until people have a basic grasp of right and wrong, they cannot have gone into acts of sin. Once that is understood however, "Man must be punished for his own sins, not for Adam's trangression".

[Mr. Smith]

Does it make sense to assign an age or time to this, or even to think of the Fall as something that happens to individuals rather than to humanity collectively?

Yes it does, because the understanding of choice and the consequences come at different points for different people. The mentally lame may never come to fully grasp it or grasp it late, the precocious may understand at a very early age, the average probably around 5-7.

Whenever that point is, is when there is the active drive to choose right or choose wrong...and since there is always but one right choice and many wrong ones (a good chunk of course appear to be good ones)...it is inherently so that at that point, someone will Fall to sin.