Leah H. Somerville, a Harvard neuroscientist, sometimes finds herself in front of an audience of judges. They come to hear her speak about how the brain develops.

It’s a subject on which many legal questions depend. How old does someone have to be to be sentenced to death? When should someone get to vote? Can an 18-year-old give informed consent?

Scientists like Dr. Somerville have learned a great deal in recent years. But the complex picture that’s emerging lacks the bright lines that policy makers would like.

“Oftentimes, the very first question I get at the end of a presentation is, ‘O.K., that’s all very nice, but when is the brain finished? When is it done developing?’” Dr. Somerville said. “And I give a very nonsatisfying answer.”