1. I have little to no lower chest definition even though it's clear that I have worked out my pecs area. I'm sorry but I can't provide pics for the moment. Is this lack of definition due to body fat or lack of lower chest workout? I tried doing decline bench but I couldn't get the hang of it, just felt awkward. My body fat is such that I can't see my abs but some of my ribs are visible if I raise my arms so I'm guessing it's around 18%.

2. I'm 19 years old and my feet or height have not changed for a while, so is it ok for me to start doing heavier workouts instead of the higher reps for growing teens?

1. Your bodyfat is probably too high.
2. Yes.

^^^ what he said.

try dips instead of decline bench if thats a problem.

Practice decline bench with lower weight. That's right, push your ego asside and practice with the wussy weight. Once your form is right, raise the weight.