For adults, it is possible for eye prescription strenghth to get better. It is indicated from the experiment that caring for your vision through diet and nutrition, good workplace habits, and UV protection can help your eyesight.
It depends. For children who are nearsighted, their prescription strength will become worse as they grow. While for children who are farsighted, the prescription strength will get better as they grow. However, for adults who are nearsighted, their eyesight get better as they get old while these adults with farsightedness will find their glasses prescription worsening. So the prescription does change throughout the life.
If you are meaning vision gets better by "prescription strength get better",I think the answer is no.The best condition for your eyes is they stabilize at a certain level and don't get worse anymore.They can't get better naturally.