This can be seen as early as age 18. It comes about from abnormal burn-up of Vitamin C and calcium caused by prolonged drug use. Skin on face has general appearance of sagging. This is most notable around the eyes, mouth and cheeks. Obviously age has to be taken into account when this is noticed.
Eyes are dull.
The best example you can get for yourself is to look very closely at people who don't do drugs and then at a person who is a known drug user. The difference is very distinct. Again look closely at people you know who don't do drugs and get a good idea of their skin tones. Then compare to a known drug user. You can see the difference.