it's just about odds. Of course if you start receiding later chances are you won't hit the highest norwood because obviously as you are older there are less time for male pattern baldness to evolve and also some say you receide slower when you age but it's possible. That's why it's interesting to look at family hairloss history(grand father/father..).
If you have a diffuse/female pattern type, that's another battle. at 20, 30 or 60 yo chances are you will finish high norwood pattern(pattern means we can see thinning pattern but you are not necesseraly bald)
final note norwood 7/ slick bald are quite rare, that's the only positive thing we can say but again we are all concerned because we just don't know even if the odds are low...