the sport dlx is most likely a late 80's/early 90's

from T-Mar's post "Late 1980s, early 1990s Taiwanese models:

Sometime in the late 1980s, Centurion / Diamondback started sourcing Taiwanese manufacturers for some of their lower level models. The format varies somewhat and I have a limited number of samples, but in all cases the first number appears to represent the year of manufacture. The serial numbers vary from 6 to 9 characters in length and may, or may not, include a letter as the first character."

based on exage, "made in Taiwan", and also the decals. and those decals are from 1986+ bikes!

edit: and the paint scheme.. always thought those were 1988+