Some do. (But you wouldn't want to work for them, anyway.) To get around this problem, at least in your resume, you're only going to take your employment history back 10 to 15 years, maximum. Unless you're a total pothead, that's plenty of time to have built up enough accomplishments to show the employer you have what s/he wants. The other way around this problem is that you don't show the years you went to school, or were in military service. The only place you'll show years, in fact is for your employment.
Remember, the resume is a marketing document that's intended to get you the interview. Once you're in the interview, you're going to sell your experience and skills. You're going to show that interviewer that you don't need to be trained. You will have done your homework on that company to know why they need the position you're interviewing for, and how you can address that need better than anybody else. Thinking that way should get you around the issue of age.