Brain plasticity, window of opportunity, it’s all babble. You can learn new languages just fine as you age; the matter here is how much time you spend using the language.

The reason why adults perform generally worse than kids learning languages is mostly motivational, and not spending enough time with the language. But as an adult you got access to a bunch of resources that kids wouldn’t, such as a decent grasp of grammar on theoretical grounds, that you can (and should) use to your advantage.

Note however that watching sitcoms will likely not be enough to get any decent grasp of any language. (Otherwise I’d be speaking Japanese, given the amount of anime that I watch.) You’ll need proficiency on four levels: hearing, speaking, reading, writing.