dismissing 15 year olds as overall unintelligent is rude, inaccurate, and detrimental to self-esteem but it is also true that the average 15 year old does not have the experience required to have sound opinions on the grand majority of things they care about
being 15 doesn't magically give you bad opinions but what it probably does mean is that you're relatively emotionally immature which is generally what people are referring to when they call 15 year olds stupid
Subaru would despise Rudeus' father and things would drastically change from there
Rudeus would try to have sex with all the village kids in arc2 and be beaten to death with rocks