According to Disneyland Resort Rules, Guests over the age of 14 are not permitted to wear costumes in the theme parks (with the exception being Oogie Boogie Bash which has specific costume guidelines). That doesn't mean that you can't still represent your favorite characters.

Adults frequently dress up at Disney parks, though they must adhere to strict guidelines that prohibit guests over age 14 from wearing full costumes or masks during regular operating hours to avoid confusion with official characters. To circumvent this, many fans participate in "Disneybounding," where they wear contemporary, everyday clothing styled in colors and patterns that evoke a specific character without being a literal costume. However, during special ticketed events like Mickey’s Not-So-Scary Halloween Party or Disney Jollywood Nights, the rules are relaxed, and adults are encouraged to wear elaborate costumes provided they are family-friendly and do not contain weapons or floor-length trailing fabrics. Additionally, "Dapper Day" events see thousands of adults dressing in sophisticated, vintage-inspired attire. While you will see many adults in simple themed t-shirts and Mickey ears, the culture of "dressing up" is a significant part of the community, ranging from subtle style choices to full-scale period fashion depending on the occasion.