There's no birthday that reliably marks the switch from picture books to chapter books. Some children are ready at six, some at nine, and pushing the switch too early usually backfires — a child who's handed a book with no pictures before they're ready often concludes that books in general aren't for them, which is the opposite of the intended effect. The better approach is watching for behavior, not counting candles.
Signs they might be ready
They ask what happens next before you've finished a page. Genuine narrative curiosity — wanting to know where a story is going, not just what's in the picture in front of them — is one of the strongest tells that a child can hold a plot in their head across pages, which is exactly what a chapter book demands.
They tolerate — or ask for — books with fewer pictures. Not zero interest in illustration, but a willingness to sit through a stretch of text without a picture to lean on.
They can retell a story's plot, roughly, a day later. If your child can tell you what happened in a book from yesterday, even messily, they're already doing the memory work a chapter book requires between reading sessions.
They're choosing longer picture books on their own. A child gravitating toward the denser end of picture books — more text per page, more subplot — is often self-selecting toward the next stage before you'd think to offer it.
What "ready" doesn't require
It doesn't require independent reading. Read-aloud chapter books are exactly how most kids enter this stage — you doing the reading, them doing the listening and following. The skill being tested at this transition is attention and comprehension, not decoding.
It also doesn't mean picture books get retired. Plenty of children happily read chapter books at bedtime and pick up picture books during the day for years afterward. Treating it as a strict upgrade rather than an addition tends to create pressure that isn't necessary.
A gentle way to test the water
Start with a chapter book that has illustrations scattered throughout and short chapters — five or six pages, not twenty. If a child asks to stop partway through a chapter and doesn't ask to come back to it, that's useful information, not failure. Try again in a month or two with the same book or a similar one. The gap between "not ready" and "ready" is often shorter than parents expect, and there's no cost to testing it more than once.