Each hair follicle has a growing cycle. Hair grows for some length of time, then stops for awhile, and then falls out and starts over. The length of the growing part of the cycle as well as the overall health of your hair (whether it splits and breaks off, for example) are what primarily determine your hair's maximum length.
As far as looking older/your age, I do agree that some shorter hair cuts can look more polished (and thus more mature) than some long hair cuts. But personally, I think the difference is in how styled your hair looks. If your hair is long, but it looks like you have put it into a very intentional style (not just left it loose), I think that has more or less the same effect as a shorter hair cut. I think the reason many people associate short hair with maturity is because, honestly, getting an intentional-looking style takes way less time with short hair, so you're more likely to see it.
Otherwise, I agree with the other advice people have given you. I don't know if I look young for my age or not (people have mostly not commented on it), but I think I look more like the age I actually am now (almost 28) than I did a few years ago. I used to wear my hair very long and largely unstyled, and while my hair was healthy and in good condition, it wasn't flattering and it did make me look younger. My hair is shorter now (although I am growing it out again), but more than that, it has a style that looks like I did something on purpose.
I also have become a daily makeup wearer, and people do react differently when I don't wear it (like on weekends). I also try to dress in a more tailored style, and pay attention to what jewelry I'm wearing. I own some "real" jewelry, and do wear that pretty regularly, but I also own some not-real jewelry that I like. I tend to stay away from anything that looks obviously faked, and even in my less expensive jewelry, just stick to things made of genuine but less expensive materials. I am a big fan of pearls, though, since they go with just about everything, look great against my skin, and are a very classic accessory. Plus, my 18" relatively-large-pearl necklace looks awesome with an open-collared shirt.