I am a young(ish) therapist and have worked with clients older than me without it being a big deal to them. I guess it depends on what is being brought to therapy and the needs of the client. I personally wouldn't choose a therapist less than 15-20 years older than me, but then that has felt right for me. It's up to the individual I think.
FWIW I have trained with therapists 20 years older than me who have less life experience than me in many ways. It's impossible to tell just from someone's age I think.