@klods hans no... both GE and GBG stay in the age you were at the time GE/GBG started. This is why we (had to) yell at people the first week of a new era release NOT to age up until after GE and GBG have both started so that the demand for new age goods won't hit treasury for 1-2 weeks. You also get to fight against the previous age troops in GE/GBG instead of facing the new age right away.
People joining a guild after GE/GBG start does nothing to that seasons GE/GBG. They are locked out.
The only thing that changes once GBG has kicked off is if someone who was there when it started leaves or is booted. Then it causes a recalculation for GBG costs on all the sectors.
and
@thelegend88 yes, if the person is in LMA when GBG starts and then ages up, the demand for LMA goods will stay until the next season. UNLESS that person leaves the guild, then it's updated right away at least in GBG. For GE, you're stuck with their costs and score for GE no matter when they leave.