Look at the officer you are talking to. Age can be difficult to estimate, but what we are really looking for here is a gauge to at least get the general idea. Now, to more specific physical characteristics. Move from the top down. First is hair.
Color is relatively easy.
Length is not. Long hair to me might seem like medium length hair to you. Try to describe it so someone else can visualize it. Sometimes it's better to describe it as looking like someone that most everybody has seen. Facial hair is next.