Is it possible to win at the olympics at long distance running by starting training at the age of 19 years? I have an ectomorph body type.

Yes! But possible & probable are two different things, not to dash your hopes. Perhaps, rather than focus on winning at the olympics, you can set a goal of personal achievement, say participating in a 1 mile event, then walking a 5k before running a 10k and then moving towards half-marathon & full marathon. Some folks like ultra-marathons. You're a winner just by trying. As nike logo says, just do it!