In my mid-30s, I'm much less confident about just about everything. I still have some medium to strong opinions, but I've been wrong enough times about things I was sure about that it's just hard to hold such strong views on things having only read it in a book or two.
There are a couple of differences between your experience and mine though, I think. The first is that I'm about to finish my PhD and have a great deal more expertise than I did in my early 20s. So as my expertise has grown, my certainty in many things, especially those outside my field has diminished. This is largely because I realize just how much more depth most fields of knowledge have and just how much is required to really hold an informed opinion.
I also deal with a fair number of young people, college students in my department and in other contexts. I recognize some of the same certainty and optimism I used to have at their age, and I hear some of the same opinions I used to hold on such things as vegetarianism, government systems, and economic policy. I remember having those beliefs, and talking to these young adults, it's striking just how little evidence their certainty rests on. It's worse inside my field, where students come in with their "great idea" having done no research and spent all of about 5 minutes really thinking through the ramifications of their idea, but that same certainty and optimism is there.
I often wish I was as idealistic and energetic as I was 15 years ago. But I wouldn't trade it for the naive surface-level thinking that went along with it. I like seeing the depth in the world too much, even when it shows just how shallow my thinking is in most areas, and even if it means that in most areas of life I'm forced to constantly face my uncertainty and lack of expertise.
Anyway, I hope that was at least a little bit helpful.