你先说说自己的理解呢？

咱们先看句子的主体结构：“Understanding what distinguishes people who battle with Alzheimer's as they age from those whose mental acuteness remains strong well into their 80s,90s and even older”是主语，“is a major focus of current psychiatric research”是谓语。这句话主要表达的是“理解某种区别是当前精神学研究的重点”。
然后分析这个主语部分的复杂结构：
“people who battle with Alzheimer's as they age”指的是那些随着年龄的增长而患有阿尔茨海默症的人；
“those whose mental acuteness remains strong well into their 80s,90s and even older”指的是那些即使到了80岁、90岁甚至更高龄时仍能保持思维敏捷的人；
“what distinguishes ... from ...”是一个关系代词引导的宾语从句，用来描述上述两类人之间的区别。