Frederick is 13 in this episode, which aired in January 2001. But on "Cheers", he was born in November 1989, so he should only be 11 years and a couple of months old!
Additionally, in "Star Mitzvah" in Season 10, Frederick is just turning 13, and having his bar mitzvah.
Didn't Frederick have severe allergies to the outdoors in "The Show Where Lilith Comes Back"?
Frasier: (Speaking of Freddie who has just arrived for a visit) He doesn't seem very happy to be here. He hardly said two words to me.
Martin: Oh, it's perfectly normal - you're his dad. Kids that age don't want to talk to their dads.
Frasier: I never stopped talking to you.
Martin: (after a thoughtful pause) I know, buddy.
Frasier: Frederick, we're gonna be leaving in the morning.
Freddie: But I don't want to go.
Frasier: No? I thought you'd be dying to get back to your Game Boy.
Freddie: Well, yeah. But I kind of made plans.
Frasier: Oh, really? With who?
Frasier: Oh, then you don't have plans. We leave at...nine-ish.
Freddie: OK, fine. I have plans with Melody.
Frasier: The s'mores girl?
Frasier: She's cute.
Freddie: She's a cheerleader.
Frasier: Ho-ho! I remember a particular cheerleader from my youth: Lorna. She was a beautiful girl. In fact, I was so intimidated by her I-I could never even work up the courage to approach her...
Freddie: Dad, I kissed her.
Freddie: I know. It was her first time. It was my first time too.
Frasier: I see.
Freddie: Don't tell Mom about this, OK? She'd ask all kinds of stupid questions.
Frasier: Don't worry, son. It's just between you and me.
Martin: When you hear that scratching at your window late at night, remember that young couple... (thrusts out his hand, with a fork sticking out of the sleeve) and Fork Hand!
(Frasier gives him a look)
Martin: Oh, come on! That's scary stuff. I told that to Duke last summer, and he wet his sleeping bag.
Frasier: Wait a minute. Isn't this Duke's sleeping bag?!
Martin: Now you're scared!
Daphne: We need a man's opinion: the velvet trim or the multi-colored? (sees) Where's Jack?
Niles: He left.
Roz: He left?!
Niles: It was an emergency.
Roz: What kind of an emergency? Like he-saw-me-and-he-thought-I-was-a-dog emergency?!
Niles: No, no, no, he said you were pretty.
Freddie: I have to go to the bathroom.
Frasier: Freddie, I...I don't see a bathroom.
Martin: You're surrounded by fifty thousand acres of it.
Frasier: Ah. Oh, how quaintly rustic! Yes, Frederick, just pick a tree and make it your own!
Niles: Freddie, about what you just saw - I know you've always had special feelings for Daphne, and there's something I need to tell you.
Freddie: I already know about you guys.
Niles: Oh. Uh, and you're OK with that?
Freddie: I liked her when I was a little kid. I'm over it now. I mean, she's like a hundred.
Niles: She most certainly is not. (Nyah Nyah!) It just burns you up that I got her...
Daphne: I mean, what could be sadder than growing old alone? (realizes) I wasn't talking about you, Dr. Crane. You've got your father to grow old with.
Frasier: Ah, yes.
Frasier: No, no, Roz, it is, actually. You see, every year we pick a book to read and discuss. It's been sort of a bonding thing between us. Gosh, you know, I hate to brag, but Freddie really is a very articulate young man. He's very imaginative, and not to mention what a great sense of humor he has.
Roz: Oh, that reminds me, Frasier! Alice said the cutest thing this morning...
Frasier: Now careful, Roz, you don't want to turn into one of those mothers who bores everybody talking about her child.
Cleo: Hi, Dr. Crane. I've been dating three different guys, and I can't choose between them.
Frasier: Is it that you can't choose, or you don't want to choose?
Cleo: It's just hard. One's really funny, one's adventurous, and one's sensitive. They're all gorgeous too.
Roz: Oh, excuse me, Cleo! This show is for people with real problems!