The variation in age when a child graduates can be dependent on the individual child (abilities and aspirations), the capacity and resourcing of the project that they attend (to continue to support through additional tertiary training or studies, or even providing vocational training themselves), and also the school system of the country they are in (some countries do not have the children starting school until 7 or 8 years old, and may require children to repeat grades before moving onto the next level).
Sadly, turning 18 years of age does not mean that they have completed their schooling, or have even necessarily had a good chance to move through school grades at what we might consider a “normal’ pace. This is why each young person’s expected completion is managed on a case-by-case basis.