Using Therin's example of someone starting a thread to ask about a book only to get that book spoiled is a good example of why we need spoiler codes more often then not. if you get into the habit of using them, then you won't have to worry about what thread you are in and if it's OK to spoil or not.
Why is it we get spoilers in threads that have (spoilers) in the thread title, yet not when there is no (spoliers) in the thread title?
Given that some people are behind and are catching up, six-months (IMHO) is too short a time. Granted, I'd give spoiler regardless of age, but if we have to have an age limit, I'd go for 2-years.