I sometimes feel pretty old here, being 20 years old now. Though like some others say, I don't think I really have the mind of a 20 year old. The one thing I've been noticing more and more of is that I've been finding myself with less and less time to dedicate to this site. Though I'd say this was especially last year, which was actually quite busy. After a few setbacks, I find this year leaves me a lot more busy being tired with the time I do spend working rather than having my hands full all around the clock.
If there's one thing I notice, it's that Serebii Forums kind of seems to have its own generations. On the games section, I can see a definite group of distinctly more mature users who kind of have their own community. I started Pokemon Big Brother in that section quite a few years ago, and managing to keep it alive allows me to take some trips down memory lane a couple of times and re-watch Endoplasmic Reticulum, The Meddler and myself bickering over things as angsty, newly-pubescent young'uns who probably take things a little too seriously.
Using google to dig up ancient, closed threads from the catacombs has also proven interesting to see. I've gotten to watch the Royal Rumbles start out as barely having any sort of rules whatsoever to being an extremely organized game with some big ol' rule books to it, all the while, ER moving on from being a relatively unknown brother of a well known player with crobat as his trademark pokémon to beginning to make his own name, with many name changes along the way (often needing to correct me on what to call him more times than he may rather), to finally becoming a really big cheese in the Rumble community. Also, I've been able to watch the reality show-style games in the Games section start out as a couple of relatively cheaply-written gimmick games to evolving into a genre of serebii games in their own right, with many different shades of the prototype, some of which actually executed with some form of good literacy and plot work. It's really interesting to look at if you try.
All that considered, I personally wouldn't say that this is a question of being too old for the site so much as it's one of which generation you fit into. There are groups of younger users who do their thing, and groups of older, more mature users with their own more mature activities and discussions.
And then people try to get in on things out of their age group, and everything gets all muddled up. :S