It is true older people themselves often complain that their memory is not as good as it once was. However, much of what we call "loss of memory" is not that at all. There was usually incomplete learning in the first place. For example, the older person perhaps had trouble hearing, or poor vision, or inattention, or was trying to learn the new thing at too fast a pace. In the cases where the older person's mind really seems to become less active, it is not necessarily a sign of becoming less active due to old age. Often it is simply a sign of a depressed emotional (压抑 感情) state.
请问1、much of what we call "loss of memory" is not that at all中的that如何理解？到底指代什么？2、 There was usually incomplete learning in the first place.这句话应该如何理解？

that指上一句中的“their memory is not as good as it once was”
我理解问题2的意思是：通常首先是因为记忆不完整
注意in the first place 是首先，起初的意思。
上文提到老年人所谓的记忆力丧失，减退并不是真相，因为记忆是多种人体机能合作的结果，他们有可能听力不好，视力不好或注意力不集中或急于记忆（精神紧张），其实仅仅是说明他们的感情处于压抑状态的表现。