I don't consider it that unusual no matter their age or the time of the year. It's not always easy to figure out why.
A molt or mini-molt is always possible. For many different reasons a hen can molt at any time, it's not always tied to age or the time of the year. If you see several feathers her color floating around that's probably the cause. A molt is not at the top of my list.
Since it is one specific hen I think that rules out an egg stealing varmint. Not sure how you can tell that it is that specific hen not laying and the others are.
High on my list is that she is hiding a nest from you. That's a pretty common cause for something like this.
It doesn't sound like she is broody.
Some hens are more susceptible to weather than others. Some do slow down dramatically in bad weather, either heat or cold. As long as she is acting healthy I wouldn't worry.