UPDATED: October 2026

Hi, I’m Mark Koester. I build tools that help people act on their own data: their biological age, their reflections, their unfinished songs. I spent five years on product and growth in consumer longevity, and learned that the measuring was never the hard part.

What I’m doing now

From 2021 to 2026 I was Head of Product and Growth Lead at the Epigenetic Clock Foundation, a nonprofit behind some of the original epigenetic aging tests. I helped take a research lab test to a consumer product: I built and ran the consumer platform and patient portal, built the software for a clinical trial on reversing biological age, led growth (consumer revenue grew about 7x in a year), and sat with people one-on-one as they worked through their own aging data.

What I took from it: the number is the start, not the point. The real opportunity is the journey after the test.

These days I help health and longevity teams with product and growth, mostly as a fractional or advisory partner. I’m also part-time Product Lead at AudioCardio, working on hearing health.

If you’re building in longevity or preventive health and want to compare notes, I’d love to hear from you.

What I’m building

  • Playback Pilot: a Mac and Windows app that helps music makers organize their projects and finish more music. It grew out of my own pile of unfinished songs.
  • Stay Reflective: journaling with AI follow-up questions. Its best moments came when a coach or teacher invited their people to reflect together. Thousands of people have taken a reflection on it.
  • Oh Sail and Yesteryard: smaller experiments, a browser sailing simulator set in Marina del Rey and a walking-history app for Venice.
  • Open source: QS Ledger for personal data and Awesome Biomarkers.

Music

I make music as Stellar Mammals. The latest album came out this year with a new singer I’ve been collaborating with.

About this blog

I’ve been writing here since 2007. It’s where I think out loud about goals and how people change, self-tracking and health, books, creativity, and building things. A few places to start:

Speaking

I’ve spoken at startup, design, and data conferences, mostly on health products, behavior change, and personal data. A few: Building Transformative Health Apps (Health 4.0 Leadership Summit), Customer Development for Startups (Startup Boost LA), Self Across Time: Time Series for Health (PyCon Malaysia), Machine Learning for Quantified Self (PyCon Thailand).

Background

I grew up in Omaha, Nebraska, studied in Chicago (DePaul) and then Paris and Strasbourg, and spent nearly a decade in China, where I ran a software agency (INT3C) and led regional development for Techstars. Back in the US, I co-directed Startup Boost Los Angeles and worked on product at a UCLA medical school spinout before joining the Clock Foundation.

I speak English, French, Chinese, and Spanish. I live in Venice, California.

Say hi at markwkoester at gmail.com or through my other accounts.