if you see any of those signs you need to arrange for a professional to come out and analyze the roof. A good rule of thumb is every 15 years you should have your roof checked
There are two options for replacing a roof: add a layer or remove the existing. Adding a layer is the cheaper option of the two. The disadvantages are your roof can get heavy, and the sub-layers might rot. Also by adding a layer you are not replacing damaged plywood.
Removing the existing roof is the safer option and it will allow professionals to give you a significantly longer guarantee. If you are planning on being in your home for over 5-6 years this is the best option. Finally in most states having over two layers is illegal, make sure you check with Roofing Professionals to see what is best for you.