It can also skip a generation. Grandpa can be bald at 30, daddy fine, and then little Jimmy goes bald.
Sometimes you will see two brothers, one with a full head of hair and other is as bald as Blockhead.
You can never be sure of when the gene may strike. I live in fear of going bald as I really don't think I could handle it , sorry, that may be weak as hell but I can't help it.