Most start to read when they're around five or six and are reasonably fluent by the time they're eight or nine. I say most - it's important not to compare a child with others or to expect him or her to conform rigidly to suggested ages. Some read much earlier, others much later. Some will become adults before they can read fluently, and some will never make it fully. People in the latter two groups are often there because of the stress put on them over their reading when they were children.
The right age for any child to be able to read is when he or she is ready, and not until.