Medical schools want someone who stands out. Because you get the same song and dance from so many of the chemistry, biology, biochemistry majors, it's easy for a humanities major or perhaps social science major to stand out through activities related to what they're interested in. When you've seen 50 applicants in a row that majored in a bio/chemical science with similar research and then there pops up a guy who majored in Economics whose honors research was on the economic impact of ethnic healthcare disparities, who sounds more interesting to interview and talk to? Adcoms are people and they can get bored of seeing the same stuff over and over again.
In addition to this, I recently remember seeing stats that showed that the average MCAT score for nonscience majors was slightly higher than that for science majors. Part of this is probably because so many otherwise mediocre applicants decide to choose a biology/chemistry major to "augment" their chances. When they ultimately get rejected, it can give the appearance that medical schools have some preference for non biology/chemistry majors but I'm willing to bet there's just so many bad biology/chemistry applicants that it had nothing to do with their major.
The age thing I think is good. Keep in mind that being older means that you have more time to make up for weaknesses in you application strength from freshman/sophomore year. It may be the case that it is this ability of older applicants to have times to "prove themselves" that gives them the benefits you perceive, and not simply that they are older. However, it doesn't seem to be the case yet that anyone is "harmed" from applying right out of college so I wouldn't get too concerned with the age stuff just yet. I think a good way to look at it is, if you decide you want to do other things before applying, medical schools are just as receptive to that decision (provided you're not flying kites in your backyard). If you imagine that someone like me would be 25 as a resident if I applied right out of college, that's a little weird for me to think about when you consider where most 25-year old males are on the maturity scale.