A:  Rachel Bilson always looks stylish and age appropriately youthful.  Sure, you can imitate her look and wear trousers with a t-shirt and flats.  How to do it?  Trousers have a wider leg width than jeans or cigarette pants, so you want to be sure to have long legs–or a heel to balance out the width.

Here’s another couple of looks you might try:
(1) Dress down your trousers with a casual boot and a slouchy long sweater for a comfy, relaxed look. 
(2) Layer two t-shirts of different colors over your trousers.  Accessorize the casual look with some fun long necklaces.  Be playful, but also be careful with proportions.

Pictured: Martin Lightweight Wool Cuffed Trouser | $69.99 (sale) at Banana Republic.