Cognitive decline is the loss of ability to learn new skills, or recall words,names,and faces that is most common as we age.

VOA: standard.2010.06.17

What this means, if you average it out, is that since children start learning their first words at about their first year of life, they learn about nine new words a day.

如果你将这个数字平均一下,这意味着,儿童从一岁左右,学习第一个词开始,每天学会九个新词

耶鲁公开课 - 心理学导论课程节选