// I have never ever claimed to know anything whatsoever about the subject of climate change... //
Even aside the fact that this total lack of knowledge hasn't stopped you from commenting on the topic in the past, what this also shows is that, despite your long life, you've never looked into the topic. What use then, on this specific topic, that you are older? Someone who was younger, but was motivated to research a topic, will know more on that topic than anybody older than them.
Which, frankly, shouldn't require spelling out. This is how research works. No doubt there are things that Thunberg understands far less well than you do, because you have looked into them and she has not. That, again, is nothing to do with age. Her age is therefore irrelevant; what matters is the message, and, as you have established, you are in no position to evaluate it.