Openness is a personality dimension that probably doesn't change much over time I'd guess. But maybe it does ...

Reading fiction is probably the most significant input for me. Moving cities/high schools during puberty was an impetus for change. Sports probably also forced me to be vulnerable to external feedback/reality.

I don't smoke weed anymore, but there are periods of my life where I did and one of the features (for me at least) is a fracturing of attention which might be related to opening one's mind. I generally describe weed as a drug for mental pivoting. Used for a short period of time it can help you figure out where you should go/act, but when I and several people I've known use it beyond that, we get stuck perpetually deciding where to go/act.

I don't think opinions should be generically strong. They should be as strong as what you have evidence for