You can take acting lessons all your life. Does this mean you will be the world's greatest actor? Probably not, but if you want to at least improve your overall skill level and performance as an actor, it is always a good idea to take acting lessons. Of course, some folks will always be awful, no matter how many lessons they take, or for how long, but lets leave them be, at least they're trying, which brings us to a good point about the value of taking acting lessons in general. If your goal is truly to become a great actor, it is only logical that you would seek out means by which to make yourself a better actor. Acting lessons are almost taken for granted by many working actors as a part of their regular schedule.
Acting lessons can be given in almost any form imaginable. One on one private tutorials, improvisation, and visualization are a few good examples. Acting lessons will help the average actor get in touch with whatever they have inside them and use their strengths to give great performances. The actor that believes that he doesn't need acting lessons will not grow as an actor, and scenes will show their lack of experience.
Overall, even if you're not even pursuing acting as a career, acting lessons are a valuable source of knowledge that can help you in your everyday life. There is such a variety of things being taught, you can always take something away from acting lessons that sheds light on life issues or gives you new skills to deal with people and situations.