elder    older   elderly

elder、older这两个词均是old的比较级，在用法上有所区别。elder本身也是形容词。 

1.elder表示“前辈的”、“年纪较长的”，仅用于同一家庭成员的比较；older则不限于此。

My younger son is five and my elder son is nine。我的小儿子5岁，大儿子9岁。 

Tom is two years older than I.汤姆比我年长两岁。 

2.elder在句中只用作定语，不作表语；older两者皆可。

My elder sister does her homework all by herself. 我的姐姐独自做作业。 

His elder son got married last week. 他的长子是上星期结婚的。 

I think his mother is older. 我认为他的母亲年龄大引起。 

3.elder只能用来指人；older既可修饰人，也可修饰物。例如： 

My elder brother joined the army when he was only fifteen.我哥哥年仅15岁就参了军。 

This dog is older than any other dogs here.这条狗在这里是最老的一条。 

I have a sister older than myself.我有位比我大的姐姐。 

It is said Mr Chen is over ten years older than Mrs Li.据说陈先生比李女士大10多岁。

4.elderly是形容词，“较老的”。elderly是委婉用语意为上了年纪的，一般不说old man而用elderly man代替，the elderly泛指老人。

His  mother's  elder  sister is an  elderly  woman now.