I'm about to interview someone who applied for the same position that I have: senior software engineer.

The candidate is 13 years older than me and has worked on the same position for more than 15 years. 

I find this to be extremely fishy and I can't figure out a way to get out of this mindset that I know is not right. 

Why is this OK and not a red flag? 
Will the candidate influence the team in a good way? Or will the candidate seem bored and show tiredness for doing the same thing endlessly?