While it's true that those in their 30s or older can be closed to new idea, that's not because they're "old", it's because they're people and some people are closed-minded.
I find a lot of these complaints are more about the cultural and lifestyle gap between those in their 20s and those who are older.
I remember the 80s. I'm not particularly interested in going bar-hopping. I find most social media to be an annoying drivel from people who vary between exhibitionists to just liking the sound of their own voices (everybody is talking, nobody is listening). I don't have that same sense of enthusiasm because for me everything isn't new. Most things really are derivative. To paraphrase something Don Draper said, I've reached a point on my life where I think I've basically met every kind of person there is. That's not to say that people can't surprise you. They can. It's just that you realize they're not as different as they once seemed.
This is exacerbated in the startup world since so many are cut from the same cloth: valedictorians in high school, magna cum laude, graduates of Stanford/MIT/CMU/Columbia, interned at Facebook/Amazon/Microsoft/Google, now working for one of those or some hot startup. It's a whittled down group of the technocratic elite who often-times don't really know how privileged and lucky they are.
At Google there tends to be two kinds of people: those who have worked in the outside world, particularly in Corporate America, and know how lucky they are and how different this is. And those that haven't and think this is just how the world is.
I don't begrudge them their successes and accomplishments but it is a form of cultural isolation--even inbreeding.
All of this means there tends to be a smaller set of common social norms with the "20s set".
At the same time, I pick up new technologies, languages and frameworks as much as I ever did. Possibly more so. Just now I've been doing a lot of AngularJS. That certainly didn't exist in my 20s. While I may not have the same youthful exuberance, I have experience and can draw lessons and parallels from programming in every stage of the Web's development (from CGI scripts on).
I will say this to those planning to have children: do it while you're young and it has nothing do with fertility. Children simply take an enormous amount of energy and commitment and this is far easier to bounce back from the younger you are.
As for the rest of it? Take another comment on this thread:
> Once somebody is fully set in their ways it's almost impossible to change their mind without divine intervention
That's fairly blatant ageism but, more than that, it's from someone who only knows people like him- or herself that wants to surround themselves with the same. Like I said: commonality and shared social norms are really what's in play there.