I recruited in tech for 20 years and now I write resumes, mostly for tech professionals. I generally start talking to my resume clients about ageism around 40 as well.

I may ask if we want to keep the oldest jobs on the resume (which tend to be irrelevant anyway) or to list graduation dates that reveal age. I go through a quick exercise with clients to show how old they are "on paper", the basic formula being that we assume you were ~22 when you graduated college (or if no grad date or college education is listed, we may assume you were between 20-22 when you started the first job on the resume). If we remove the oldest job(s) and delete graduation dates, we can often mask age a little bit if that is desired.

Many clients decide not to try masking age, as any company that might discriminate based on age isn't a place they would want to work. I respect that as well.