The number of overdoses in Maine’s southernmost county has risen sharply, and the heroin users are younger now. Suzanne Michaud remembers vividly the night last November when two police officers showed up at her home in Kennebunk to tell her that her daughter had been found dead of a suspected drug overdose. “What hurts the most was I wasn’t there to save her,” Michaud, a retired schoolteacher, said through tears.