David and I didn't plan this but, on the heels of Glenn Davis' age 28 season we have Jim Deshaies' 29 age season which, coincidentally happened the same year, 1989.
That year Deshaies had arguably the best season of his career posting an ERA+ of 117, with 225.2 innings pitched and a 15-10 record. Helping to fuel the team to the 86-76 record.
I'm sure many consider the 34 year old Mike Scott to be the best pitcher on that 1989 squad. He won five more games, pitched 3.1 more innings and had 19 more strikeouts. However his ERA, ERA+, FIP and WAR on both Baseball Reference and FanGraphs, fell just short of Deshaies. Does that mean that Deshaies was the clear better pitcher, no, but it also doesn't mean that Scott was the better pitcher either. Both have strong cases for the best pitcher on the '89 team and the difference between their statistics is insignificant. Let's take a look at their Game Scores.
In 32 starts Scott averaged a 59.5 Game Score, Deshaies average a 57.68. Anything above 60 is a good pitcher and anything above 50 means the pitcher gave their team a better than 50% chance to win the game. So Game Score goes to Scott, but it's worth noting he made two less starts than Deshaies did.
Breaking down individual games, both pitched five 80+ games, four 70+ games and from there it deviates. Scott had more 60+ games, but Deshaies had more 40 games and less 20 and 30 games, both had 10, 50 games.
Now that I've got your head spinning what it means is that both had the highest of highs, Scott had more highs, but Deshaies avoided the lows Scott had. Again, it pretty much comes out a wash on who the better pitcher was in the '89 season.
With my family in the process of moving to Germany during that season I have no insight into what the perception of that season was. I imagine that Scott, taking into account his career, was considered the ace and best pitcher on that staff. I suspect that one or several of our dear readers will be able to confirm or deny my theory. Whatever the perception was I think Deshaies was just as good if not, maybe, slightly better.