I am curious what counts for “mid career”, he started coding ~6 years ago. Presumably he is in his 20s. Here him talking about leaving philosophy: https://www.philosophicalhacker.com/2014/04/22/why-im-glad-m... Edit: LinkedIn ~agrees with this estimate
As the age of retirement is currently ~67 (depends on the country but unlikely to go down in the future) and we assume you start your career after college in your early 20s - let's say 22 - you get ~45 years of work.
Let's say the above is closer to a maximum than an average & "generously" go with a round 40 years of work; Then mid-career would be in your 40s.