I've always thought that hair is not relevant for defining someone's progress. I'm from Europe, and I have some friends with a "hairy" ethnic lineage I guess, and they already have decent hair amount everywhere even though they're only 14-15 years old, probably only about 2-3 years in puberty?
I have another question. What's about feet growth, when do they grow the most? How do they develop?
The way the human body develops, there are a set of systems which each develop in a certain pattern. However, the timing between the systems is not nearly as fixed. That is why the Tanner Stage Calculator asks so many questions. Some are asking the same thing from a different point of view to judge the consistency of the answers, but most are focusing on the different systems. Some systems can run ahead or behind other systems.
When it comes to hair, the color of a person's hair makes a big difference. We tend to rate things relatively. Therefore, someone with light skin and dark hair can look like they have more hair than a light skinned, blonde hair or dark skinned, dark hair person, even when all have exactly the same amount of hair.
As examples, leg and arm hairs tend to start developing in stage 2 and are definitely present in stage 3. But chest hair tends to start developing in stage 4 and is definitely present in stage 5. Pubic hair starts near the base of the penis in stage 2 and then spreads outward in a pattern that finishes by the start of stage 5. Yet each of theses systems have a little bit of give and for some people a particular system may start sooner or later.
Now also factor in that you normally would only see a guy's leg and arm hair most of the time, so it would be hard to really determine what stage they are in overall. You can't go by age because some are early developers and others are late bloomers. I know some boys who were in stage 5 at the age of 15. I knew one boy who was in late stage 4 at the age of 12. But there are others who have just entered stage 2 at the age of 14.
Hands and feet grow the most in stage 2, arms and legs in stage 3, and the trunk region in stage 4.