Can a driver who was drunk and underage and left the scene of an accident, claim that the passenger is at fault for "hitting the steering wheel"?

Well hit and runs are never legal, period. If the passenger was equally negligent by hitting the steering wheel, it may not be an excuse but it is certainly a factor in determining who was at fault. Further, an underage drinker is already committing multiple criminal acts and in some states, a passenger who may also be impaired may be violating the law, as well. If you or the driver have been arrested or charged, you may wish to each speak with counsel to protect your respective interests in the investigation or civil lawsuit. Bottom line, nothing is ever certain until the evidence is in and the evidence has to be pretty clear that the passenger was equally as liable as the drunk driver to relieve the drunk driver of some (not all) liability.