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 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.