I lokked it up the best example I could find was Deathstroke Annual #1 from 1992 where Dick states that no member of the Titans is older than 21.
Then there is a newspaper Article in "New Titans Annual #5" (1989) it is said that Nightwing is 21, but it is hard tell if you can take that serious.
And New Titans #71 also kind of implies that he is 21.
To me it looks more like this Action Comics issues was the outlier, especially since it had to be happen more than 2 years after COIE, for Dick to be 22. And doesn't really fit into the timeline in universe.