The New York City's narcotics officers are being told to stop arresting suspects over the age of 40 — a major strategy shift designed to target younger dealers, who are more likely to carry guns and use them, The Post has learned.

Top brass issued a directive that makes it all but impossible for cops to bust older drug suspects, in order to combat a spike in shootings — which are up 7 percent in 2015 compared with the same period last year and 12 percent over the last four weeks, police sources said.

The new policy was laid out in a May 14 memo obtained by The Post that scolded police bosses for busting people outside the 18-40 demographic — and demanded written explanations for arrests of midlife perps.

Division commanders who got that memo immediately ordered the rank and file to stop making collars in that age group and threatened officers with transfers and other discipline if they did not comply, according to multiple police sources.