That's true, but people who were not born then are registering there in large numbers today. Many of the members won't even know it is a "Forum" they're on. It's just a place where they can find out about stuff that is important to them right now.
Yes, I agree, but I find "seasonal" forum examples a bit like cheating.
Seasonal forums are like leeches. They don't provide much within themselves in core, they leech off of the main product from "outside". What does this mean?
Every year millions of young people apply for school/universities and have exams. So this creates a lot stream. So there is a season for it when young people look out for info on the internet, and this forum leechess off of that.
Or other ones are sport forums, like football/soccerr. So when it is match day, you see activity and during the season. And when the season ends, the forum activity drops down and the cycle begins again.
So, yes, people register in large numbers on there, but if you look closely, the numbers are misleading. Most of the accounts are there once or twice and you'll never see them again when "their season" ends a.k.a like when they have applied to a university or whatever. So bringing up them as example is like cheating, cause the numbers are hyperinflated. But still what you say is true of course, it is just as an example not a good one cause again, it is the exception to the rule. Most forum owners don't have such "evergreen seasonal forums" like school stuff. That cycle never ends, there will always be new people grow up and go to school, so you will always have an audience. But for example if you have a forum for the latest call of duty (or whatever), well, good luck when that game dies and the next one is out, cause its "season" is only for a limited of time.
So most forums either have such short-lived seasons like games or they are outside of that "evergreen" stuff.