Depends on what you consider as male pattern baldness.....
If you consider a Norwood 1.1 to be male pattern baldness, then yes, nearly 100% of guys in their late 20's and above have male pattern baldness. However, it is a well-known fact that NW2 is reached by practically all men, regardless of whether they lose any further hair or not. So I'd consider male pattern baldness to be anything significantly worse than a NW2 (Norwood 2.5 or worse). While the NW2 or "mature hairline" is certainly male pattern baldness, it is a minor degree of male pattern baldness which nearly all men experience, and it is not cosmetically significant. Since this minor degree of male pattern baldness is so common, it is considered normal for adult males. And some may say that male pattern baldness is male pattern baldness, and is inexorable - but the fact of the matter is, the non-balding NW2 guys do NOT recede any more throughout their lifetime, not to any measurable extent, even if "the rest of their life after reaching NW2" is half a century. male pattern baldness progresses at different rates in different people. While stable NW2s are quite common (and represent non-balding mature men), its not very uncommon to come across a stable NW3 or NW6. (Although at NW6, there's not much left to lose!). NW7 is a stage which usually only those with early, rather rapid hair loss reach.
So if you only look for Norwood 2.5s and above, the figure would be a little less than 90%.
Cheers,
Arjun