Casting age

We are looking for a middle aged man(40), a fairly old man(56) and some teenagers (from the ages 13-16). The Age of these characters is important to the story and we will only be accepting the best people for the production however we need to consider our budget and how we are going to get the best people for our extremely small budget. The likely hood of us getting well known actors is minimal, so we are going to look for our actors inside of school to reduce the price and availability issues.