I work in a mechanical engineering design role in the UK, I love design and I'm not much of a people-person so I would be happy staying in the bottom end of the company which focuses on the applications of engineering to a project rather than roles in management of employees or liaising with other businesses. I have no salary ambitions either so non-manager salary is good for me.
I have been told by friends that companies hire the design engineers with the idea that they will progress up to management so if I'm 35 years old and I haven't moved up the corporate ladder then nobody will be interested in hiring me. They say that the junior roles are for young energetic people while the more senior roles are for people with a lot of experience in the industry, as I get older I won't match the jobs I'm looking for.
The last thing I have heard supporting this is that if I have 15 years experience and I'm not searching for a very senior role then I'll be overqualified for the jobs I want and a hiring manager may think I'm looking for a temporary job at his company because I'm applying to a job below my worth.
How can I ensure that this doesn't make me unemployable if I refuse to move into managerial positions?