Like a lot of other statements in this article, this is a gross generalization. There is absolutely no reason for an older person who is fit and healthy, mentally and physically can't learn as fast as younger people. There are plenty of examples of retired people doing PhDs just for fun. In fact many retired people do their best work in retirement (if semi-retired) because they don't have the financial stress or any pressing need to 'please the boss'.
If there is any slowdown in mental accuioty as one gets older, I'd say that is more than compensated for with experience and accumulated wisdom. The only aspect of software engineering that might discourage older people from sticking with it is that the high level of stress due to long hours, being on call, and so on. can take its toll on blood pressure and general well being: older people are more savvy about their health and so often deliberatly pursue less stressful careers to optimize their life expectancy.