Maintenance Program

I'm a 65+ man whom has added 10 pounds of muscle in a few months of doing the Starting Strength method, after being at a stagnant weight despite regular gym workouts on machines for years. I'm still intermediate, but my question is, once I reach my physiologic plateau, how do I structure a "maintenance program" to keep my gains realizing that I will not be able to increase weights indefinitely? Any recommendations on frequency of workout, intensity/volume changes needed to maintain man strength?

What do you mean by physiologic plateau? Do you mean your genetic potential, beyond which you cannot get any stronger? Most people won't ever reach this point as it requires extremely dedicated training with no setbacks. If you are referring to the point at which age prevents you from adding weight to the bar, you'll likely want to see exactly where you are at when that happens. You'll want to look back on your training and see what you liked/didn't like, what lifts moved along easily/which ones did not, how often you now want to train at that point, etc. There are then various options where you can deload certain lifts a bit and build back up, possibly in a different way, with some different exercises and also doing a bit more accessory work to manage systemic fatigue.

Maintaining is very difficult, you should always work to increase intensity by varying volume, frequency, or exercise selection. Ex: 3 day HLM vs 4 day old man Texas or DUP. It is also difficult to provide programming advice with the information provided. Fortunately for you, there is a book written specifically for your demographic:

The Barbell Prescription

Ricardo
I am replying based on my own experience at age 71 with few years under the bar. You are probably aware that at your age you are fighting against sarcopenia, lower testosterone and reduced metabolic efficiency. I found that I reached a psychological plateau rather than a physiological one. The struggle to continue to "train" became something I did not enjoy or look forward to, so I transitioned to a 2 day/week maintenance program, usually alternating between sets of 5 and 3. One day is squats, press and lat pull, the other day is bench press and dead lift. I now enjoy the workouts, knowing that I don't have to worry about adding weight to the bar, although I will occasionally push myself a little.