Know how to interview. Dye your hair. Keep relevant. Keep healthy. Have a niche. Have a plan b. Keep a good attitude.
The interview process will change. They know you have the ability to do the job. Questions are more around of do you give a shit, directly or indirectly.
Grey hair changes peoples mindsets about you, you can stop dying it once you are secure in the job. Or, just shave it all off. Dress young.
Bragging about cobol skills just annoys people and are next to worthless at most jobs. Know the latest do-hickey your co-workers are playing with, at least so you have something to talk about, even if your experience tells you the do-hickey is otherwise a waste of time.
Other people here talk about the niche.
Many older co-workers I know are incapable of doing the job because of bad exercise habits, energy levels, and other health issues. Stress oozes out and is sensed by others.
Have a plan b, a business on the side or simply make sure work is optional. Desperation makes for a strange interview and worse co-workers.
Most people are capable of doing any job, if they care. A bad attitude from a person skilled at dealing with others can be catastrophic. People have to pretend to care, especially for self-hypnosis. "If a prostitute can learn to pretend to enjoy her job, so can you."
Most people I have worked with older than 35 are failing at least one of these, if not several. Thing is, everyone fails a few of these, but they are more obvious in older workers. A bad eating habit is troublesome in a 20 yo, but nearly killer in a 40 yo. An attitude problem from someone skilled at manipulating others (because of years of experience being manipulated) can destroy a team.