First thing first. What country applies for your access? Each country will have their own considerations, and possibly different conclusions.
Second, there may be an official rating by a ratings board if required for that region. The rating might be self-applied if there is no such requirement, like a best guess by the distributor. Additionally, there may be unofficial groups that also give age recommendations (e.g. Common Sense Media: the green checkmark).
For TV shows, the conclusion may be different per episode. So how does that reflect the show as a whole? 18+ might apply to at least one of the episodes, 16+ might apply to most of the episodes, and 14+ is what CSM thought about it.
These are user-reported figures on IMDb, and might be inaccurate or replaced with later ratings.