Combining a grey windbreaker with dark green chinos is a wonderful pick for a casual yet seriously stylish outfit. Send an otherwise mostly classic outfit down a less formal path by finishing off with a pair of beige athletic shoes. This look is probably what you've been looking for as a teenager.
For a cool and casual look, make a teal flannel long sleeve shirt and dark green chinos your outfit choice — these two items work really well together. If you wish to easily dial down your look with shoes, introduce beige athletic shoes to the equation. Gents who wonder how to wear cool casual outfits as you work your way through your teen years, this combo should answer your question.