When Rocky leaves to go to Russia to fight Ivan, his son is about 8 or 9 years old. When he returns not even a year later, his son is around 15 years old.
very noticable slipup. although i haven't checked the credits, i was thinkin maybe they had the same person play the son in rocky 4 and rocky 5. The 5th one wouldn't be as good if it were the same kid from the 4th one though.