This section covers the things that happen to a child on a timetable of their own — the arrival of teeth, of words, of walking, of bladder control, and of feelings too large for the child holding them. What an adult does in response sits next door under parenting skills; the division is worth knowing, because it decides where anything is filed.
One idea runs through everything here and is worth stating once at the top. Every age given in this section is a range, usually a wide one, and the midpoint is a description of a population rather than a target for an individual. Development is also uneven across areas, so a child well ahead on one thing and unremarkable on another is following the normal pattern rather than making a trade-off. For a child born early, every age is counted from the due date.
Where that general reassurance stops being useful is the short, specific list of signs that are worth raising rather than waiting out — and the most important of those has no age attached to it at all, since the loss of a skill a child already had is worth a conversation whenever it happens. That list is set out under baby milestones, which is the anchor article for the section and the one to read first.
Alongside it: teething, which accounts for a good deal of the first two years and for several things that get wrongly blamed on it; potty training, where current specialist advice differs sharply from the familiar version; and toddler tantrums, including the episode that frightens parents most and is almost always harmless.
In practice the material gathered here concentrates on the first three years, which is where this kind of question clusters and where the published guidance is clearest. The ages beyond that have sections of their own — children, and then teenagers — and the developmental stages belonging to them, puberty among them, are covered there rather than here.