When should I switch from steel to graphite shafts in my irons? Good question, but a not so simple answer. The usual reasons to switch from steel to graphite are to play a lighter shaft to increase swing speed, or to reduce the amount of vibration from a steel shaft. But that logic is a little outdated.
If you are playing a set of irons that is more than say 8 years old that have stock shafts, more than likely they have 110 to 128 gram shafts (Nippon, KBS, or True Temper). In the last 5 years or so, most manufacturers are offering very light weight steel shafts from 85 to 105 grams in their stock steel iron sets (XP 95, KBS Tour 90, Nippon 950GH). The truth is, I can find both graphite and steel shafts that run the full spectrum of weights from 75 grams to 125 grams, so it is not a question of weight that should drive your decision to move away from steel. Sometimes irons that feel too heavy can actually be too stiff of flex or maybe the swing weight has changed if you re-gripped your clubs with either a larger or smaller grip. If your clubs feel too heavy, you should have a good clubfitter help you determine the proper weight, and then, try some shafts in both graphite and steel at the same weight to see which gives you the best distance, accuracy, consistency and feel.
The same thing applies to reducing shaft vibration. There are Sensicore inserts available for steel shafts that reduce vibration. There are also grips that absorb more vibration (Winn, Golf Pride CP2). Graphite shafts definitely reduce shaft vibration so if you have arthritis or chronic tendonitis, this may be a good option.
Some golfers have concerns that going to graphite shafts will cause more inconsistency. This is really an issue with getting the incorrect weight and/or flex of the shaft than it is the quality of the shaft. I see a lot of golfers that purchased graphite shafts in their irons only to find out that they were way to soft (flexible) than what was stated on the shaft. Again, a good clubfitter can help you not only find the right shaft for your irons, but build them to the exact specifications that will help you get better distance, accuracy and consistency.