Although Mayaguez is  medium sized city, there is not such thing as nightlife.  Locals frequent La Tertulia-bar or Oyster bar mostly  on Fridays and Saturdays, you will find a mix crowd from early 20 to late 30s.  For adult couples and "singles of any age" looking for companions, try the dance terrace of Holiday Inn Hotel which is a nice place to dance with locals and meet ladies (see note below).   Also, the Joyuda area at Cabo Rojo  has a lot of seafood restaurants where you can either drink or have a decent dinner (mostly couples and families). For the young crowd, try Rincon's suffer bars such as that near to the lighthouse, El Tambu, Sunset, and JD's in Aguadilla. If you are fluent in Spanish Aguada's pubs such as Bungalow & Riverside Castle are a good bet. (Rincon, Cabo Rojo, Aguadilla and Aguada are neighbor towns located relative close to Mayaguez, at least for the locals who are  desperate for action). 

Note for the gentlemen of any age:  If you want to impress the ladies at night wear an ironed long sleeve shirt and leather type shoes.  No sandals please! As anywhere in the world, men are judged by their shoes.  You can wear any type of pants as long as they are not short.  Locals usually wear jeans especially dark jeans at night.  It may sound strange to you but t-shirt underwear is a turn off for most of the chicks. It really doesn’t matter how hairy you are. Finally, a cool water type cologne is a must. Good luck and have fun!