Based on age:
* Adult
* Mature
* Experienced
* Elderly (depending on the age range)
Based on behavior:
* Mature
* Sophisticated
* Adult
* Responsible
* Serious
Based on legal status:
* Adult
Based on style or appearance:
* Sophisticated
* Elegant
* Classic
* Stylish
It's important to consider the specific context in which you are using the word "juvenile" to determine the most appropriate opposite.