One thing that older people (50+) have impressed upon me is that learning gets harder as you age. It's not just that your body and mind become tired more easily. In general, if a 20-year-old and a 50-year-old spend the same amount of time studying something, the 20-year-old will do better on a quiz. At least, that's what older people tell me. I'm your age, so I haven't experienced this firsthand.
If at some point your rate of learning becomes slower than the rate at which relevant technologies change, it's not possible to stay relevant. Your deep understanding won't do you nearly as much good if you only know languages and frameworks from 15 years ago.