Trends in subgroups can indicate a different trend. The above comment is not about a linear trend. It's about an average statistic for a population based on a sample that's not representative of the population age distribution for a process that probably isn't linear across ages. If the older half becomes symptomatic 100% of the time, and the younger half becomes symptomatic 0% of the time, it's very misleading, though not technically incorrect to say people become symptomatic 50% of the time. There's a huge impact on how to handle the problem.
https://en.wikipedia.org/wiki/Simpson%27s_paradox