Why is there a 5 year gap on your resume? It sounds like you didn't sit around twiddling your thumbs... you built stuff during that time. On your resume, treat it like you were working a job and talk about what you built. Highlight your open source contributions and if possible, tie them to your resume. Sure, some people will treat an ex-founder as a negative, but many will see it positive. You only need one job.

There is definitely ageism in tech, but 39 isn't old. I'd be happy to take a look at your resume and provide advice if you give me a way to contact you. But it sounds like the issue isn't your resume... indeed you are getting lots of interviews, so maybe it's something you are doing in the interview process. Do you have a sense of where things go wrong? Are you often getting to the final stage before hearing no?