Of course, you can still watch Spongebob Squarepants even if you are 18 years old (or older.) There is no rule that people will stop watching cartoons once they hit puberty or for that matter when they are 18 years old. You might be shocked that there are jokes said in the Spongebob Squarepants show that are meant for the older audience. So it will be more fun for you to watch the show and see the jokes that you missed as a kid.
Also, nowadays Spongebob memes are a hype in social media platforms. You can either create your own soon-to-be-famous memes or get to watch what episodes the trending Spongebob memes came from.