The genesis affect was influencing the planet. It also influencing the life that was on the planet, causing it to evolve into plants and animals that we know of. Spock landed on the planet when life started to evolve, and that affected him. It caused him to regenerate; it didn't make him from the ground up. Since his being didn't contain protomatter, he was completely stable. The genesis planet became unstable because its energy was stretched out over the entire nebula and created a new star. Spock's body regenerated while the life was evolving and the planet was, and Spock was also aged. He was aged back to about 55, and Kirk took him away. Because he wasn't on genesis any longer than he was, he wasn't aged any further. He then aged at his normal pace from there.Only if he was still on the planet and he was not at the time.