As someone who is rather significantly vertically challenged (ie. quite a bit shorter than 5'), I definitely feel your pain. I'm 22, and people seem to constantly think I'm around 13-14.

The way that I deal with this is by putting a huge amount of effort into my personal style. I try to dress nicely, I style my hair, I wear quite a bit of makeup, etc. I've found that when I walk around on a daily basis with smoky eye makeup, curled hair, and high heels, people seem to believe me to be around my actual age. But I've been doing this for so long that I'm comfortable with it, and over the years I've developed a passion for makeup and fashion.

But yeah, people definitely treat you differently when they think you're younger compared to when they discover your actual age. It's kind of amusing to see their faces when they realize it, actually. Or maybe that's just me trying to be more comfortable with the fact that I'm really insecure about my height.