“We’ve had 105 years of supporting girls and a girl-only safe space,” said Lisa Margosian, chief customer officer for the Girl Scouts, who added that the organization felt “blindsided” by the announcement. “So much of a girl’s life is a life where she is in a coed environment, and we have so much research and data that suggests that girls really thrive in an environment where they can experiment, take risk and stretch themselves in the company of other girls.”

For families involved in scouting, the announcement led to a scramble of questioning phone calls and frantic Facebook discussions as parents and scout leaders wrestled with tentative plans for the transition. Next year, girls will be allowed into the Cub Scouts program, which had been limited to young boys. A program for older girls is expected to be available in 2019, giving them a path to earn the prestigious rank of Eagle Scout.

The Boy Scouts has offered girls limited access to some programs before, but it has never before welcomed them into its core Cub Scouts and Boy Scouts programs. And the introduction of girls still does not mean that Boy Scouts’ gatherings will necessarily include both genders. The smallest groups of Cub Scouts will continue to be single sex.

Michael Ver Duin, a third-generation Eagle Scout and troop leader in Grand Haven, Mich., said he was “super excited” for the change — an opportunity, in his view, for girls to have the same choices as boys.

“The closest I’ve had to a negative comment is ‘I know there will be some who do not welcome this,’ ” he said. “The two biggest things I see as a challenge to explain to parents are, ‘What kind of problems are you going to have if you had boys and girls on the same camping outing?’ and ‘Do boys need time to be boys and girls need time to be girls?’ ”