age has lots to do with drawing ability. you can easily figure out someone's skill based on what they can do at a certain age, this works particularly for younger people. In the end art takes time, talent and practice. some people are naturally more adept to do good art than others, some people pick it up more naturally, others have to work harder at it. The average 17 year old won't draw like him.
take it or leave it, that's how it is. I have a bachelor's degree in art, and do art full time for a living. I know what I'm talking about.
take Joe Maduriera for example. He started drawing comics PROFESSIONALLY for Marvel at the age of 16. That's a lot of hard work, self discipline and natural talent.
so yes you can judge how someone is doing with their art by their age...because in the end if you don't put the work in it you won't get better, then you turn 30 you realize that you never worked on it that hard and that's why you aren't that great. being a good artist is a balanced blend of time spent, how your brain processes artistic information, hand eye coordination, and loads of hours of actually applying your knowledge.
and ramonra, that drawing is pretty good, but in my opinion laziness is no excuse for poor art. that's when self discipline kicks in. you should have worked harder on the clothing. His hair is too sketchy, doesn't have flow, the eyes are looking in two different directions, and it looks really rough still. finish that shit up if you want it to look professional. and for a 15 year old it's okay, but I was already drawing better than that at 15. also about your friends drawing extremily well...how well something is drawn is only relevant to what you think looks good. Something that looks good and something that looks professional can be two completely different things. you gotta have standards to begin with to create good art.
hell this age phenomenon you can notice in kindergartners as well. have some children draw figures, and see the differences... Some kids will draw a circle for face, some kids will draw cheekbones in, some kids will go as far as to represent the ears of the people, it is the latter that have a more natural nack for observation and recording said observation in their drawing, defining natural artistic ability.
if you don't believe me that I do art, look at my sketchbook (everyone seems to nonchalantly ignore it) the link is right under my sig....