What is a nice way to let someone down romantically?

I went on a date, which went well, but I figured out right away that she’s too young for me and that having a relationship wouldn’t work for me. She’s 24; the number doesn’t bother me, it’s that she’s still pretty wet-behind-the-ears, a bit naive, and is generally not in the same place as I am in life.

She is a really nice girl, though, very sweet and earnest and I’d like to remain friends. She seems to be very interested in me and I am quite flattered, but I need to make it clear that I only want to be friends.

I ask your advice because I tend to be overly direct and I am not good at all at sugar-coating things (but am interested in learning how). All I can think to say is, I really enjoyed hanging out with you, but we’re in different places in life, so I don’t think a relationship will work. I’d like to be friends, though. Is there a kinder way to say that? Or a way that doesn’t sound condescending? I don’t want to seem like I’m full of myself or that there was anything wrong with her.