It’s worth looking at this picture of a normal distribution: https://en.wikipedia.org/wiki/Normal_distribution#/media/File:Standard_deviation_diagram_micro.svg You can read the Wikipedia article it comes from (https://en.wikipedia.org/wiki/Normal_distribution) but that gets into serious maths pretty quickly.
Most natural characteristics follow this including, for example, height. Most people are neither especially short or especially tall. Being a little shorter than the average or a little taller than the average is pretty common but being very tall or very short is rare.
I would expect the timing of puberty to also follow this. There is quite a lot variation around the average but being really early or really late is much less common.
On seeing a doctor, what the medical profession seem to do with very many things that can be measured like this, is assume the middle of the distribution is fine, maybe two standard deviations away from the mean, but consider the extremes worthy of investigation. That doesn’t necessarily mean there is something wrong but that becomes increasingly likely as an explanation as just “normal variation” becomes less likely as the explanation.
