I'll bet that every one of the people who fails to take you or your opinions as seriously as you feel they should knows exactly how it feels because they went through exactly the same thing when they were your age. In fact, that's why some of them do it to you now; it's a rite of passage sort of thing, and they feel that if they had to go through it then you should too. Builds character, or something. I'm not saying that's right, but when you get right down to it that's what motivates a lot of their behavior.
I can also think of a few other reasons you might get these sorts of reactions. One is that here may be some issue with how you present your opinions. Like it or not, it is your responsibility to understand your audience and convey your views in a way that is convincing to them. In this sense it's not so much an age thing as a culture thing; an older person from "outside the culture" who didn't present things "the right way" would get the same reaction. Consider what happens when a sales guy or an exec walks into a room full of engineers.
Lastly, we come up against the fact that older people do have some advantages over younger ones. For example:
Despite all that, there's a lot to be said for fresh perspectives and youthful enthusiasm. Your ideas may seem flighty or unreasonable to some, but a wise man once said that because reasonable men don't try to change the world all progress depends on unreasonable men. My point here is that you can't expect them to subscribe to your idea of "merit" without some justification. Just as they have ways of doing technical things and won't change those without good reason, they have ways of doing social things and won't change those without good reason. Show them the reasons. Anticipate their objections, address their concerns, and show them in terms they will accept how things would work better if they were more accepting of ideas from "people like you".
BTW, I'm 35. I get flak from both the youngsters and the oldsters. It's like "no man's land" in the battle between generations.