They can know the technical details about depression as an illness, yes. As far as life is concerned, of course they won't have as much life experience as someone older. It depends what you want from them an how good they are at their work. I think therapists who really are very good listeners, who know their job very well, who know when to speak up and when to shut up and especially how to facilitate YOUR therapy experience, I believe those can help you no matter their age or yours, if you feel comfortable with the age gap enough as to trust them or learn how to. In my experience with therapists of different ages, age didn't make any difference; the therapist's own therapy and self-awareness and the quality of their training did.

That said, I can see what you mean. I'm fairly young myself, but even so, I wouldn't seek support with some of my issues from someone who is in an earlier stage in their life. Sometimes when I seek support, not just in therapy but generally, I long for a kin of empathy and understanding which comes from experience ... not necessarily the same experiences exactly, but the same opportunities. There's something important about age when it comes to understanding the real extent of some issues, I believe, and I mean this in a way much deeper than just listening and empathy for emotions expressed can go.

So, I'd say this depends on what you need most ... a good professional (could be of any age) or that kind of "I've been there' or "I know many peers who have been there" kind of understanding and shared experience. But mostly, I recommend you just see how it feels being in therapy with your T. Does the connection feel genuine enough to you? Can you trust them? Are they ethical? Do you have chemistry enough to work together? After all, if this is not the right therapist for you for whatever reason, you WILL know.