In my country there is a lot of talk about this subject. For many professions the age goes down more dramatically than others, and thirty is already considered old for many things. It has been said that changing careers after forty is suicide.
There are many examples on both sides of the argument, with many people training for new careers in their fifties as they accept buy outs in my area. You can watch it unfold in real time and see some people fail and some people succeed.
Personally, I think that it would be a good thing if people became their own bosses. If a person works for themselves, there is no question of getting hired or fired. By the time a person is mature, they should have picked up at least a skill or two that they can turn to their financial advantage, and not be dependent upon an employer for their financial security.
Of course, some people can't work for themselves for many reasons and must seek out an employer. There are quite a few corporations that like to hire older workers. By the time an employee is fifty or so, they usually have gotten into the practice of showing up for work (and on time). Over the age of sixty-five workers are usually happy to work part time without benefits, which younger workers with children can not.
Every age has its good and bad points. Many types of work are done better at certain ages. Construction is for young men, their foremen/supervisors and employers are usually mature. Skills increase as physical strength decreases. As long as the mind is stronger than brute force, an old person can work forever.
A person who at 46 or older is worried about employment has already made too many mistakes or developed too many flaws. The older we get, the more valuable we become. If you don't, you are doing something seriously wrong. It is true we may not have the stamina we had in our youth, but our mind should be working four or five times as well as it did in our youth. We should by then have accumulated a body of knowledge that is unique unto ourselves that can be translated into some type of income.