I'm 20 so obviously there are no therapists younger than me, and I prefer a young therapist (in her 30s or so) just because it's easier for me to relate and vice versa. My therapist just turned 30, I think our age difference is perfect because she still remembers the pressures of university and can empathize with me, but she has a decent amount of experience, or at least enough for me.
In a similar vein I can understand why someone older than me would want an older therapist than themselves. It makes sense.