Pros and cons of having a Ph.D. advisor who is (possibly) younger than me I applied for a Ph.D. position. 

A new assistant professor who would join that university next year reached me and asked whether I wanted to work with him. I have been working for two years since my Master graduation. I think his direction suits my interest. 

He began his Ph.D. right after undergraduate study so I guess he is probably at my age, 27 or 28, even younger. If we are both 40s, I might be ok with that.
As you get older, it is basically dead certain that you will at some point be reporting to someone who is much younger than you are.  Think about why that has to be true.  If you're still stuck, ask yourself how old you think your supervisor will be when you're in your 60s.  You need to get over it.