First of all, 40-45 years ago, which I remember clearly, old people talked about the weather much more than young people did. As old people now are simply yesterday's young people, the question is what happens to increase that interest?
I can think of a few things:
1) Old people are retired, so they don't have work to talk about. Don't you talk more about the weather on the weekends?
2) Old people often have physical pain or discomfort related to the weather, so it is like being a runner with an injury coming on, or feeling better, he talks about it.
3) Old people forget more things and get more facts wrong that when they were young on many topics. Weather is simple and right in front of them, so much easier to be correct in the comment. It's also something that was around when they were young and is consistent with long term memories.
4) Old people value the basics more, such as being alive for another day. The story of the day is sunrise, sunset and the weather. It's on their mind, so they talk about it.
5) Light, warmth, a cool breeze, rain, thunder are like youth and energy. They get it externally, when the internal supplies are dwindling. Younger people can alter their environment much more easily by physical activity, such as warming up with a run and feeling the breeze along the way. For the older people, the changes are from the weather.