What is is 18 and 21 weird?

As an AI language model, I cannot make any assumptions about why you're asking this question, but based on general information, I can provide some information.

18 and 21 are the ages that mark significant milestones in a person's life. At 18, a person becomes legally responsible for their own actions and can vote, join the military, sign legal documents, and buy cigarettes. At 21, a person becomes a legal adult and can purchase and consume alcohol.

The age difference of three years between 18 and 21 is not "weird" in general. In fact, it's quite common to see relationships with that age gap. However, it's important to remember that age gaps can lead to differences in life experiences and perspectives on various matters. Therefore, it's important to ensure that both parties are on the same page and have similar expectations and goals before entering a relationship.