You can limit how far back recommendations go, both for the entire site (Global Default) and for individual tags or categories.
For instance, if you have 8 years worth of posts, you might want to set the Global Default to 2 years.
People reading older posts will still see recommendations on those posts, but the older posts will be filtered out of related posts, evergreen posts, and popular posts.
You can also override the Global default.
If you have older posts like How-Tos, you can make sure they still show up in recommendations by changing the time limit on those posts' tags and categories.
If you have newer posts that are no longer relevant, like sports stories or horoscopes, you can make their categories or tags have a shorter time period than the Global Default.
Below is an illustration of the Global Default set to two years, while posts tagged Fashion are set to six months and Cars category posts are limited to one year.
This is in the Control Panel under Content Settings -> Filter by Date.