My observations lead me to believe hair density is pretty much the best indicator of what a guys final norwood will be.
My grandfather had a "full head" of hair until he was 40, meaning no major bald spots or temple recession. However, his hair was gradually thinning all over the top probably from age 25 onwards. When he died at age 58 he was quite bald on top at NW6.
Hairline recession seems to happen quickly... and often happens at a young age which tends to freak a long of young guys out. But if the density on top is good, it will probably take decades for those follicles to fully die. Some guys lose a lot of hairline early but stabilize and keep the rest for life.
That does not seem to be the case for people who are starting to lose topside density... usually once that area starts to thin... it will be gone a couple decades down the line. The guys who hit NW7 before 40 most likely had low hair density in their teens.