Changing a job every month is too often :) But seriously, it heavily depends on many conditions. For instance, if you work mainly in big corps, you may work at one place for 10 years. If you work in startups - well, many of them don't live too long. Also it depends on your level and so on.
I started to work as a Java engineer full-time in a software dev company about 2014, and worked there until 2018, when I started my own company. That means I haven't really feel any impact on my working history, as my new employer is me. BUT I know cases, when people of both groups - who works on one place for many years, and those who didn't - have troubles during career search.
As a CEO, I consider it more positive. When you work on one place for many years you focus only on your realm, you stay in your comfort zone, you don't learn (well, you can claim that you learn everyday, but actually you learn things you work everyday).
Age is a crucial question. Many talented freshers can't get a job for their level as they are considered "too young". I think age is a "good" reason to discriminate and should not be considered.
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