The term watershed is often used for this. From Cambridge Dictionaries:
watershed noun (BIG CHANGE)
[Uncountable] an event or period that is important because it represents a big change and the start of new developments:
a watershed event/moment
The discovery of penicillin was a watershed in the history of medicine.
The origin of its figurative usage appears to be from the geological term, where a ridge or other geological feature separates flowing water into different drainage systems. Thus the metaphor is very similar to a crossroads or fork in the road, but perhaps with less implication of a choice of path (the water doesn't choose which way to go, just like "history" didn't choose in the example above).
So for your example
After that happened George's life would never be the same; he'd reached a watershed.
would especially make sense if the event "that happened" was not of George's choosing or possibly if George did not foresee the consequences. You could also say that
[The thing that happened] was a watershed for George; afterward, his life would never be the same.
or, even more simply,
[The thing that happened] was a watershed in George's life.
Note that a watershed can be good or bad, both in the causal event and in consequences.