Does physical competency really matter in desk jobs? If not, why employers usually define "require age not > 45 or 49" in most adverts?

Age discrimination happens, for young and older people. Some of the reasons, maybe besides ability, that HR professionals screen by age have to do with the following:

They don't want to hire someone who will retire soon. I hate to say it but it will come up. You need to make more promises to a company to get those high level job changes late in your career. Be prepared for this question.

Too much experience. In the eyes of HR professionals, middle and top level professionals have an issue where they are seeing really experienced candidates but could also just hire other professionals for a lower salary. Apply to jobs you can do and excel in at your high level career.

Technology: Technology is holding back a lot of older professionals. Being able to learn and keep up with certain technology being used at a company is unfortunately a huge issue for hiring managers.

Generational rifts. Sort of like hiring young people who are switching jobs a lot and demanding certain work environments, its equally hard to serve older generational work needs. HR teams are trying, but sometimes failing, to keep up with offering benefits, work environments and salaries to all these different professionals AND build teams that merge well across generations.

Someone younger can be hired for less. I said this already, and it happens, so strategy is really key. You need to be doing job hunting differently from younger and mid level people. Not all opportunities will be the ones for you. Look for the companies that match well and that want and need expert level experience.

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Its all about general ability, even when you are doing a desk job, you need to concentrate on what you do and also work properly in the field you are trying to work in. Sure age is a number, but as per experiences in middle east and where there is stress and people exercise less and full of economic and even family issues, its hard to find some one in an age over 45 who can work like a 30 years old. I said 30 because younger than that is too young for an ideal job!

In my opinion, the advertisement mentions such things, due to the general feeling that people above the age of 45 are incapable and not mentally fit (natural aging process of the brain) for the desk job.  It is not physical competency that matters to them. 

But personally, I feel that every individual should be given the chance.  The organisation can test the mental abilities of the person if it is after a particular age, to identify the really potential person with the experience to do the job.

For this, the mentality of the management has to change, which no one has the control of, except themselves. 

Let us hope for the best.

All people want to see perfection in all work. There should not be a limit, but an energetic person with experience could be older and over 49'.