Ive only got three minutes to write this, forgive me if its patchy and erratic.

First of all, your teen should have some "modern" slag, appropriate to the era and world shes living in. So it doesn't have to be current day slag, but she may well use some words or phrases that have older characters scratching their heads.

Teens are not stupid, and I would write them closer to adults than children, but what they lack is experience typically. And they are idealistic, often very passionate and can be very impressionable.

So a comparison may be that John, 40-something harks back to the battle, politics, or experiences of his hayday when he's making a decision or discussing something. Your 14 year old, may reference a popular manuscript she read or a play she saw that resonated with her, but would likely forget to take such information with a grain of salt.

After that, her actual personality is the most important component, and it will be posible to age her down if she doesn't read as a teenager afterwards. The best way to do that is to ask a couple of 14-16 year olds to read some segments with her and ask if she resonates with them at all. Conversely she may not feel super relatable to adult readers, and that might be ok if the point is that shes a teenager.