How to dress more my age?

When you say you're not dressing your age, I think of a girl that's just wearing jeans and a t-shirt. It's simple and a really childish way of dressing. Unless you describe exactly what your style is, I can't specifically help you, but here are just some general pointers.

The difference between kid fashion and adult fashion is the complexity. While a kid will put on a skirt and a t-shirt and call it a day, sophisticated fashion means layering and experimenting with different patterns and prints as well as different types of footwear and jewellery. Start with a basic item (ex: a skirt) and find a top for it. If you're more into dressier styles, find a cute blouse, or if you're more tomboy, wear something more casual. Then layer with a jacket, coat, cardigan, long sleeved top (it can be really lightweight or you don't even have to wear it when it's really hot), then look for unique pieces of jewellery to put on and some interesting shoes. If you decide to layer with a jacket, experiment with different colours and prints and which ones work. If you need help, ask a fashionable friend/sister/cousin whatever to help you find out which looks best on you.

This'll take some time for you to develop on your own, but eventually, you'll be able to find whole new outfits by yourself.