Menarche is a milestone, not the start
By the time someone has their first period, puberty has usually been running for two to three years. Breast buds appear first (thelarche), then pubic hair, then a growth spurt, and menarche typically arrives about two years after breast buds.
The average age in the UK is 12–13, but anywhere between 9 and 16 is within the normal range. Genetics, body weight, ethnicity, and stress all play a role.
The first year is meant to be messy
For the first 12–18 months, the HPO axis is still calibrating. Cycles are often anovulatory — bleeding happens, but no egg is released. This means cycles can be short (21 days), long (60+ days), light, heavy, or skipped, and none of that is necessarily a red flag.
By roughly two years post-menarche, most people settle into a more predictable cycle length. If cycles are still wildly irregular, extremely heavy, or absent for more than three months after that, it's worth a GP visit.