I'm 20, weigh 135, and am just a hair under 5'7" I'm reasonably small boned. When I put my hand around my wrist I can get my thumb to cover the nail of my middle finger. Also when I put my hand around my ankle I've only got about an inch of extra space. I have a really minimal amount of body fat, somewhere around 4%. My extra weight probably comes from my long torso. I'm also fairly wiry (lumpy and bony, not bulky).
I've weighed 135 since my sophomore year of HS. I was a swimmer, not a runner. Unusually enough, my weight didn't change when I started running... My muscle just shifted from my shoulders to my legs.
Highest I got up to was 140 and I was trying my ass off to gain weight (weight room every day, stuffing my face with food... I also drank a huge glass of water before I weighed myself just so I could say I hit 140 at some point in my life).
I've done my best running at 135 and worst running at 135 because I haven't made any effort to change my weight since I've started running. If you're a fair amount lighter than average and don't have much body fat, losing weight would probably be detrimental since you'd be losing either muscle or dehydrating yourself.