I'm 6 ft and between 140 to 150 lbs, at 19 years old. Will I ever fill out?
Very much; YES! Without continued training, expects that "filling out" to not necessarily be advantageous to any athletic endeavor.
I ran Track & Field for 8 years between 16–24 years old. I ran Cross Country every year between those seasons. At that time I could run 400 meters in ~40.8x seconds and a mile in 4:52 seconds. After I retired, it took three years for me to move from 152lbs to 202lbs. That was 20 years ago. I still cannot get below 195lbs, yet I couldn't run a 7 minute mile if my life depended on it.