Ageism is illegal, but it still exists in the process of reviewing resumes. As a 50-year-old who has worked in the same position for 20 years, how can I avoid giving away my age on my resume?

Most times, you don’t need to include your age on your resume. It was more common and expected to write about your personal information, like date or place of birth long ago. But those days are gone, and now, you can easily remove your date of birth from your resume.
There’s no sneaky way about it. Also, to help your resume seem charming and exciting, you should use recent language and seriously avoid outdated words and sentences. There’s not much more you can do on your resume to prevent ageism because eventually, they’re going to see you, no matter how you write your resume.