We can't tell. The figures you gave, 5' 8", and 162 pounds aren't enough for us to know if you are at a healthy weight or not.
There are several reasons for not improving that might or might not apply to you. One is that you have reached a plateau. It could be you're doing everything right, and you've hit the best speed you can at your age.
Your weight could might have something to do with it, or it might not. It could be your weight is fine, but you are training too much, too hard, too little, or not hard enough.
I would recommend trying to find out more about this before trying to change your weight. You can start by asking your coach for some time to talk.