In principle everyone can learn to meditate. So the question is less about “who” than it is about what considerations each person should take into account when considering meditation.
While there is nothing intrinsic to meditation that would prevent learning it at most ages, I believe that following the instructions provided requires a minimal level of reading skill. My expectation is that one must be able to read at a high school level.
In addition there must be sufficient personal maturity that you can apply the exercises regularly and you can learn through introspection rather than requiring instruction from another person. My goal is to have a web site to support the process of sharing between meditators. I hope that this sharing will provide a level of personal instruction which cannot be achieved through this pamphlet.
With the exception of considerations for psychological issues and the very young, anyone should consider at least trying meditation to see if it suits them.
With regard to youth, most teens and certainly pre-teens, will need personal guidance by someone who is sufficiently expert. Meditating is about learning both attention and patience. For the young this can be an overwhelming challenge because of the manner in which the process is communicated and the simple challenges of sitting still. A responsible adult should provide guidance to prevent the young student from losing interest or heading in the wrong direction.
Meditation is an exercise in attention, so people who are working through personal issues with a counselor should discuss meditation before beginning and continue that discussion after you begin meditating regularly. Consider sharing this document with your counselor.
While there are faiths for which certain forms of meditation are an important part of the religious process, meditation itself can be described, and developed, individually outside of any religious context. At a fundamental level meditation is nothing more than an exercise in attention, and patience, which does not require reference to religious symbols or terminology. The fruits of meditation are of course applicable to all aspects of life, both religious and secular.