Most children who stop reading were readers first. That's the part parents find hardest to understand — a child who worked through books at seven is uninterested at nine, and nothing obvious happened in between. The common causes are specific and most of them are reversible.
The two points where it usually happens
Around eight or nine. Reading stops being a thing you learn and becomes a thing you do for school. Books arrive with questions attached, with a reading level printed on the spine, and with a record of how many you've finished. The activity is the same and the meaning of it has changed completely.
Early adolescence. Time gets scarce, homework expands, social life takes over, and reading competes with things that are more immediately rewarding. This one is partly unavoidable, though it matters a great deal whether reading was voluntary before it got squeezed.
Difficulty mismatch
The most common fixable cause.
A child reading books slightly too hard experiences reading as work. A child reading books well below their level is bored. Both stop, and both look like disinterest from outside.
The signal to watch is whether they finish things. A child abandoning book after book is usually telling you the level is wrong, not that they dislike reading.
Losing the choice
Children read far more when they pick the books. Reading lists, levelled schemes and well-meant parental steering toward better books all reduce reading volume, consistently.
This includes the books adults don't rate. Comics, graphic novels, joke books, the fourteenth in a formulaic series, something aimed at younger children, the same book over and over — all of it is reading, and all of it builds the fluency that eventually makes harder books possible. Volume matters more than quality at this stage, and it isn't close.
Reading becoming measured
Reading records, book counts, levels and reward charts all move the point of reading from the book to the score. Some children respond well to this. Many quietly stop choosing to read once it's being counted, because the counting is now the reason, and the reason belongs to someone else.
The read-aloud ending too early
Most families stop reading aloud when a child can read independently, often around six or seven.
That's early. Being read to lets a child meet stories and language beyond what they can decode themselves, and it keeps reading associated with something other than schoolwork. There's no age at which it stops being worthwhile, and plenty of eleven- and twelve-year-olds still want it.
What tends to bring them back
Removing the measurement. Handing back the choice completely, including choices you don't like. Finding the right difficulty rather than the right book. Keeping the read-aloud going, or restarting it. And letting a fallow period be a fallow period — a child who doesn't read for six months has not stopped permanently.
What doesn't work
Requiring it, bargaining with it, or reacting visibly to what they choose. All three make reading something they're doing for you, which is the position it needs to get out of.