In my opinion the five most important computer applications students should know are: Word or some kind of Word Processor, PowerPoint, Excel spreadsheets, Publisher, and Access.
I believe the most important application is Word. Students could be taught how to use Word as early as probably 1st or 2nd grade elementary school. With technology advancing so fast, every student at all levels will need to learn how to type using a Word Processor. The next most important application I feel is PowerPoint. I have made tons of powerpoint presentations throughout my years of schooling and will make many more as a teacher. It is an easy way to introduce information to people. The appropriate school level for powerpoint is probably like 4th or 5th grade. Next is Excel. Excel lets you create lists and sorting them. It is great for categorizing information. Students will probably understand this program in the later Middle School years. I have not used Publisher much in my schooling, but the time I did use it, it worked out very good. I made a Civil War brochure in High School, and there were many boxes where you just click and type, a lot of space for pictures, and the final product turned out pretty good. Everything is laid out for you so a middle school to high school student should be able to handle this program. The last application Access I have very little knowledge of. I was introduced to it in a high school computer class. It has a lot to do with business information. You can make detailed lists of product info, client info, etc. You can then sort through the lists, send them to different places. I can honestly say I have never used it other than the time I messed around with it in my high school class. So the student level would have to high school to learn and understand Access.