Yes, it's called aging, and it's an extremely common and normal occurrence. Your player is at the age when many pitchers have already reached their prime, and they begin a steady decline.
Here's a good random example. Take a look at what happened as Walt Terrell reached his late-20s and then early 30s. His ERA and HR9 increased, his SO9 decreased significantly, and while he still had a couple of good seasons, he was never quite the same pitcher as he was at age 27.
https://www.baseball-reference.com/p...errewa01.shtml