Correct. Assuming senior still means someone who leads/mentors; then yes, I have a hard time seeing a 25yo having the kind of perspective needed. I'm turning 40 any day now; not the Travis Boober kind of 40, the real one; and imagining me leading anything at 25 is a scary thought.

I'm going to agree and disagree. I agree with life experience but some from the men and women from the armed forces are very strong leaders at young ages. I was self employed before the age of 25 with a small number of employees but what I learned is that a stronger leader will usually never follow a weaker leader so depending on who you are leading you may or may not survive in the position. Your interviewer may be thinking about who is on the team while deciding. And you will most likely have a shorter rope until respect is earned. Personally I would hire anyone at any age as long as they produced the desired results ages 16-99+

I started my professional experience at 18 I have indeed been through the struggle of proving myself to older members of the team who at first disapproved. Although now I feel like I know how to handle these situations.

Yup, there it is, life experience. Just think how much more you will have in 10 years. Another thing you want to decide for yourself is...do i want the senior dev title, the senior dev position or the senior dev money and why. The answer to that could set you free.

I have to say yes, I have been mentoring and in fact leading the project. I can't disagree with the fact that hiring a 25 yo as a senior is scary.