As a software developer, how can I ensure I remain employable after age 35?

When I hear something like "only young people work on computers", I envision someone in their 70s who thinks their grandkids are tech wizards because they can program the DVR. Actually being in the field, it's more about young people being cheap. Can't afford five experienced engineers? Hire one and four interns. My best advice is to solve expensive problems. This works regardless of age.

It's age 50 in the Quora question, why change the title? But in either case, the answer is to never stop evolving, never stop learning.

Be nimble and open minded, learn new languages, watch and discover new technologies, mentor people (don't be a land baron), and most important of all, DBAD (Don't Be A Dick).

The guy you mentor may become your boss, and so might the guy you are a dick to.

In the tech sense, DBAD typically means be nice, and don't be a technology nazi. There is no "perfect" technology for a solution. If the team wants PHP, and PHP works as a solution, DBAD, make it work for the team and don't force Ruby or Java down their throats.