Graphite shafts took over in the 1990s for drivers and fairway woods. The shafts are lighter than steel, and transfer power better on longer clubs than steel does. Also, the lighter weight allowed the club designers to make bigger driver heads with higher moment of inertia (MOI), while keeping the "hanging weight" (total ounces) of the club the same, or making it lighter.
As for irons, it depends in part on what you swing speed is. Lots of over-50 golfers I know have also gone to graphite shafts in their irons. In the St. Louis area Senior Olympics golf last year, the 60-65 age group was won by a guy who had 85-gram graphites in his irons. He was 1.5 clubs longer in his graphite-shafted irons than me with my steel. If you have a moderate to below-average swing speed, consider graphite for you irons.
Newest idea: Shafts of carbon fiber, which has a higher tensile strength than steel. (Note: CFs are a bit pricy). Also check steel fiber (check out Aerotech).