I prefer therapists who are older than me. I don't want my therapist to feel like a peer. Fortunately, at 28, I can indulge in this preference pretty easily. However, age isn't my only condition - I would not see a therapist who had been practicing for less than 10 years, I think, regardless of age. Experience in my particular issues is also definitely something I looked for when I was searching for my current T. I didn't want to be shocking or freakish to my T, and that seemed less likely if the T was a few decades older and had been practicing a long time. I think the minimum age difference I'd consider is 15 years.