The use of "would have thought" instead of just "thought" in this context adds a hypothetical or speculative element to the statement. When someone says, "I would have thought he was fifty years old," they are expressing a hypothetical assumption or expectation that, based on the person's appearance, they might have naturally assumed the age to be fifty. Using "would have thought" emphasizes that the speaker's perception was different from the reality, creating a sense of surprise or contrast. It's a way of conveying that the speaker's initial assumption, based on appearances, turned out to be incorrect. The use of the conditional "would have" suggests that the speaker is reflecting on what they might have expected under different circumstances or appearances.

"I thought he was 50" means the person had already considered this and already had this opinion. "I would have thought he was 50" is an incomplete third conditional. The speaker never considered the dad's age before finding out that he is 60. But if they had thought about it before, they would have thought he was 50"

"Would have thought" suggests that based on how someone looks, you might have guessed something, but you've realized you would've guessed wrong after finding out the truth. It's like saying, "I would've guessed he was fifty, but I found out he's sixty." It adds a layer of surprise or unexpectedness to your assumption.