Probably just age. Unless it continues I wouldn't worry.

The zucchini needs water. It’s wilting. Wouldn’t be surprised if the parsley is thirsty too.

Someone else told me that the yellowing may be caused by over watering. When I stick my fingers in the soil next to plants, it doesn't feel soggy nor parched dry, so I'm not sure whether it's caused by over-watering or under-watering.

Since these plants are so young, their roots are not very deep yet. Is it better to water more frequently but not too much each time, or given them a good drench and water only once or twice a week? Thanks!