I've gone over this a few times, trying to piece together Spock's resurrection:
Stage 1: Spock's corpse comes into contact with the lingering Genesis effect (which doesn't explain why the casket still remains) and undergoes cellular disintegration - that's if Spock hadn't already been melted down into cellular residue via atmospheric entry.
Stage 2: The cells regenerate and undergo rapid development, presumably skipping the fetal stage (since there was nothing to serve as a womb on the Genesis planet) and forming an infant Spock.
Stage 3: Spock begins rapidly ageing, until his cells reach roughly the same age as they were when his original body died.