There are people out there who think it's childish for grown-ups to enjoy YA novels. Sometimes they even write to advice columnists about it. I started reading YA as a child, true. When I was ten or so, I still loved Nancy Drew and Animorphs. But each book took me maybe twenty, twenty-five minutes to read. I needed something more substantial, for the sake of my attention span and my parents' wallets. My dad tried by giving me novels such as Anne Rice's Vampire Chronicles, but she had this terrible habit of interrupting the awesome vampire action with boring sex. Then I discovered the YA section of the library, starting with WILD MAGIC by Tamora Pierce. I've never outgrown it because there's nothing to outgrow.
I like books about teenagers. No, I'm not a teenager. But I like books about gay Chinese dragon tamers*, and I am not gay, nor Chinese, nor a dragon tamer. If the only books I could read were about lower middle class, white, twentysomething technical writers from Texas I would be very bored. YA is just another type of book. Yeah, it's marketed mostly to teens. I love science fiction and that's marketed mostly to men. About the only genre I read actually marketed to me is romance. I can deal. If the person observing me read can't, why should I care?
My policy: read what you like. Read how you like. No shame.
If you like YA, MG, children's books . . . the only childish person is the one concerned about someone else's reading habits.
*I am sadly not referring to an actual book. But I would read that book in a hot minute.