A lot of philisophical points like this took a long time to develop (universal suffrage). Even if it seems "obvious" today, most people are just parroting what they've been taught and don't give it much consideration.

Why don't you use the opportunity to address the points intellectually rather than silently thinking they are wrong?

First of all, the statements are very different. One of your acquaintences advocated prohibition, while the other merely opined that the overall results were negative without supporting prohibition.

What are the dangers in each position? How do the arguments apply to other questions, like voting age? How do the arguments apply to other groups, like citizens/noncitizens or felons/nonfelons?

I'm not an expert here, but I know there are some tough questions here that you might find difficult to answer. And then you might better understand how someone might hold a misguided position.

Really, most peoples' thinking is muddled on these issues because they haven't studied them. The only thing that keeps most people from saying such things is politocal correctness.