BOSTON (AP) — A study published in the journal Neurology says NFL veterans who started playing tackle football before the age of 12 are more likely to have cognitive difficulties after their careers.

The study published Wednesday says researchers tested 42 former players on their short-term memory, mental flexibility and problem solving. It found those who picked up the sport before they were 12 functioned about 20 percent worse. Robert Stern of the Boston University School of Medicine says both groups scored below average on many of the tests.

Stern says there is a known period of critical brain development that happens around puberty.

He says the research does not lead to any simple solutions.

The study only looked at former NFL players; the conclusions cannot be generalized to a broader population.