It might be argued that everyone should be forced to study philosophy to a certain level but as to whether everyone can learn it this is another question. It is clear that the vast majority of professional are unable to do so.
What kind of person can learn it? I'd say one who is curious, at least averagely intelligent, scrupulously honest, loves reading and thinking, is a little arrogant, is an iconoclast, has the courage to follow the logic wherever it leads, is good at self-study, is too lazy to study theories that do not work and who is reluctant to go anywhere near a university philosophy course.
Age eighteen is not too late, nor is age seventy-eight. Philosophy is like a musical instrument, it's never too late to derive some benefit from studying it. if you mean study to become a professor then eighteen is probably just in time, but if you mean study so that you come to some understanding then the pressure is off and you can start anytime. The former task is enormous, the latter more manageable. For me it was age fifty, so plenty of time yet.