As stated, a juvenile hairline at age 25 does not in any way guarantee you will not succumb to male pattern baldness at some point later in your life.
Some men simply never lose any of their hair to male pattern baldness. I saw a man who appeared to be in his 80s earlier today who had a thick Nw1. I also know a guy who was a Nw7 at age 20. Its all down to your genes.
Ethnicity is a big factor in the prevalence of male pattern baldness. Caucasians are the most prone, followed closely by Afro-Caribbeans. Asians have a much lower incidence of male pattern baldness, and aborigine australians even lower. Native Americans are pretty much immune, assuming they are full-blooded.