Parents ask us this question more than any other. The short answer: children develop at wildly different rates, and a one-year spread in math readiness is completely normal within a classroom. That said, there *are* broad milestones worth knowing about — not as targets, but as checkpoints.
By age five (end of preschool / kindergarten entry), a child typically can: count objects up to twenty with one-to-one correspondence, recognize written numerals to ten, compare "more" and "less" for small groups, and recognize basic shapes (circle, square, triangle). If a child cannot count ten objects reliably by the start of kindergarten, that is worth a conversation with their teacher.
By age seven (end of first grade), a child typically can: add and subtract within 20, understand place value to 100, tell time to the hour and half-hour, and skip-count by 2s, 5s, and 10s. Fluency with addition facts up to 10 + 10 is the foundation for everything that follows.
By age nine (end of third grade), the child typically knows their multiplication facts up to 10 × 10, can multiply a two-digit number by a one-digit number, understands basic fractions (halves, thirds, quarters), and can solve simple word problems involving two operations. This is the "if it's going to be hard, it starts getting hard now" year.
By age eleven (end of fifth grade), the child should be comfortable with: operations on fractions (adding, subtracting, multiplying, dividing), decimals to the thousandths place, basic geometry (area, perimeter, volume), and introductory concepts of ratio and proportion. This is the launchpad for middle school pre-algebra.
If your child is persistently a year or more behind one of these markers — not just on a bad day, but consistently over months — it's worth asking the school for a screening. The phrase you're looking for is "formative assessment"; most schools offer one without the full special-education process. Early intervention in math is extremely effective, and the stigma attached to asking is entirely in the adult's head.
A few more milestones worth keeping in mind, lightly. By the end of grade four most children should be at home with multi-digit multiplication, basic division, and equivalent fractions. By the end of grade six they should be comfortable with operations on decimals, percentages, and the idea that an unknown can be a letter standing for a number. None of these are finish lines — math is recursive and almost everything gets revisited in middle school — but they are the load-bearing skills the next layer leans on.
Two reminders for parents. First, ages here are approximate; an autumn-born child may reach a milestone six months before a same-grade summer-born child purely because of school start dates, and that is fine. Second, "ahead" is not always good and "behind" is not always bad. A child who is two grades ahead in computation but has skipped the conceptual work underneath will eventually hit a wall — usually at fractions, sometimes at algebra. A child who is six months behind on a fact family but solid on number sense will catch up the moment the fact family clicks. The shape of the understanding matters more than the calendar.
If something does feel persistently off, the most useful thing you can do is ask a teacher to describe what they see. Teachers spend their days comparing children of the same age across many classrooms and have a much better calibrated sense of what is normal than any individual parent does. They will not be alarmed by minor variation, and they will be straight with you if something looks like it needs follow-up. Reach out earlier rather than later — most issues resolve quickly with the right small intervention.