I want to know which are the oldest languages in the world? And within those, which is still in use?
I think a simple google search will reveal the answer. There is a fairly detailed answer at Ask A Linguist; this is particularly nice because it examines some of the issues that make the question difficult to answer. Either Sumerian or Egyptian is the oldest written form, and that the answer for spoken languages is undefined. Yahoo takes a different approach to the question of written languages, and I think their answer is splendid. "All spoken languages today are equally old." The author also notes that Egyptian competes with Sumerian for the title of oldest written language. Wikipedia concentrates on written sources and asserts Sumerian.
Languages are not static things; they evolve with the times and the needs of the users. For example, you could try to say English is about 1500 years old, but modern speakers cannot understand English from that long ago (unless trained to do so), so it just wouldn't be true.
Part of the definition of a language is that people who speak (or write) it can understand each other. I personally can't even comprehend Shakespearean English, so for me the language isn't really any more than 400 or so years old. Icelandic, which has a reputation for being the most conservative known language, is in a similar situation to my own Shakespeare with its 800 year old sagas. So probably no living language is significantly older than 700 years.
If instead you try to count modern languages and their predecessors together for the purposes of determining age, then just about all languages are likely the same age. While it does happen, it is exceedingly rare for languages to just sprout up with no predecessors. So by this definition nearly all human languages are the same age.