Wrong question because until truly old age with mental deterioration, one's age reflects one's accumulated over time body of knowledge and mental skills. I still learn new complexities all the time that are built upon decades of knowledge and understandings, especially math and science. I'm also very athletic and active outdoors that vitalizes my whole creaturely body beyond just the brain.
The lifetime static IQ narrative has been proven to be mostly false in recent two decades. Due to the way the brain changes with neural plasticity, we are what we experience and do especially the more whatever is repeated. Thus potentially one's brain can continue to improve by filling it with appropriate growing knowledge. I'm much more intelligent in some ways now than EVER when younger while it is true that some mental gymnastics were easier when younger. Valid correct knowledge is like a complex building structure that grows over time with correct additions. Fill it with science and fact and it becomes better. Fill it with false information and garbage and the result is garbage. Over time from non-use or non-experience, that neural plasticity fades away. Although I learned to read and write C language software code from merely reading books at age 30, because I haven't done so for years, would require cracking open books and reviewing. But that would take much less time than when I began because neural pathways for that learning still exist and merely need some reviving use.
One can retain mental abilities by regularly exercising one's brain that becomes most important as seniors. How and what one does so is also important. As we age into our middle ages, our brains can also develop various age related issues. For instance not being able to recall names, or vision issues like presbyopia. Thus at some point, decline of mental abilities will occur that limits how far one can grow knowledge structure.