My advice is not to strive to 'dress older' but to formulate your own style into a more sopisticated look.
It sounds like i have your *exact* body type (i know what it's like to be thin with wide hips *sigh*) and i suggest you invest in a great pair of dark wash Straight jeans (and pants). It's more edgy than wide and bootcut, and looks more sophisticated and put together than skinnies. Also, will be a LOT more flattering than wide pants (i shudder at the thought of my hips in wide leg pants).
My next piece of advice is to stay away from any type of printed or screened tshirt. Try out silk tops with fun but elegant details.. such as this Marc By Marc Jacobs top.. It will also look GREAT on your shape!
http://shop.nordstrom.com/S/2951306/...bo=2378705&P=1
If you want to wear a tshirt go for something like this Michael Stars top insead! They always come in great colours, and fit wonderfully! (other great brands for simular things are Vince and Ella Moss) I swear by these basics.
http://shop.nordstrom.com/S/2933645/...bo=2378705&P=1
I think having at least one or two GREAT jackets/coats is also a must. Something that fits YOU perfectly.
For shoes, try to stay away from very cheap brands.. they tend to look wayyy too young and cheapen a great look. Opt out of flipflops, Uggs, Converse, and basically any other shoe that could be grouped in with 14 year olds. A nice heel always helps.
I adoooree these!
http://shop.nordstrom.com/S/2936964/...bo=6007118&P=1
As for accessories- have fun but don't go overboard! Pick one standout piece like this very vintage inspired necklace-
http://shop.nordstrom.com/S/2948340/...bo=2377737&P=2
and go tiny on everything else. Studs instead of dangly earrings, one or two small bracelets instead of an arm full.. You get the idea.
Hope this helps!