Why is Linda Cardellini the only older one?

The problem with hiring actual kids who are high school age is that there are laws limiting how many hours a day they can work. That can be a big problem on shows where there are a lot of kids, especially ones who don't have much acting experience and might need a lot of takes. If you don't get the shots by the time the kid has to leave you have to come back for another day of shooting. By hiring someone older they wouldn't have to be constantly watching the clock.

She was still playing high school age kids at the time she was cast, and she absolutely passes as a 16-17 year old. In her scenes with Sarah Hagan (Millie) I don't think you can tell there is an 8 year age difference between them, or a 4.5 year age difference between her and Jason Segel.

Unlike other actresses on high school shows, Linda was perfectly believable as a 16-year-old.

Linda was the oldest but not the only one older than her character. Franco was 20, Busy and Segel 19, Seth, Martin and Neil 17. Only Millie and Sam were their age. She looked much younger. She used to play teenagers only because she looked like one, she wasn´t getting adult parts.

Casting are open for everyone as long as they look the part. So if they´re adults that look younger is much better for production due to the reason afore mentioned by other posters.

Paul Feig on writing the character:
..... and saw as I was walking around—it was pretty desolate—this group of surly-looking high-school girls walking and smoking. I was watching, and there was this one girl in the pack. I remember she looked back—she didn’t look at me, but she kind of looked away. I just saw her face, and she looked like this sweet poser that was with the group. I remember going, “Wow, that.” Because I had never had a sister. I was an only child. So I knew I wanted to write about myself and my nerdy friends, but I knew I needed to represent the freak group. So when I saw her, I was like, “Oh, I like that character. I like the idea of this girl who looks like she’s trying to fit in that way.” That’s where Lindsay came from. All the other characters are based, pretty loosely, on people I knew, at least in the writing.

Paul Feig on casting Lindsay (or Cardellini) and the others:
I think Linda [Cardellini] was the first person to come in where I was like, “That’s her! That’s her!” I think it was one or two days into the casting process, where she walked in, where she kind of looked like the girl I saw in Wilkes-Barre. When I write, I have to have the image of somebody in my head. Since all the other people were real people, I had them in my head. I was writing to their voices and their faces and stuff. But she was the one I had to make up in my head. It was really a weird moment, because she was exactly like the girl I made up in my head, and acted just like her. It was really this moment of “Holy *beep* It really blew me away. I mean, everyone really liked her, but I was like, “No, you don’t understand. That’s the girl, that’s the girl, that’s the girl.” I was obsessed.


One interesting fact is that Lauren Ambrose was up for the role of Lindsay, too. And she was great; I’d just so pre-decided that it was going to be Linda. But she was awesome.

Source: http://www.avclub.com/article/paul-feig-walks-us-through-ifreaks-and-geeks-ipart-72057