You still make GH. Even though the amount decreases as growth stops, the pituitary continues to secrete growth hormone. Increasing the amount of growth hormone your body needs once growth has stopped will only cause trouble. Growth plates fuse in response to pubertal hormones. Increasing growth hormone won't keep them open longer.
No. Igf-1 and growth hormone releasing factors can stimulate gh release in people with certain types of gh failure. They are not useful in normal teens.