Slower workouts as you get older?

Hi! I'm new to this forum. I swam competitively in college and continued to swim on my own for exercise since then. I'm now 43 years old. I don't compete; I've always been kind of a workout swimmer and measured myself based on the intervals I can make and how fast I can repeat a distance consistently on a particular interval. I've spent many years training on my own, and the last few years working out with a group. Up until I was about 40, I pretty much stayed the same pace and was able to make certain intervals on my own, and could make even harder ones when swimming with others. I've noticed that in just the past year, I've gotten significantly slower. I can't make the intervals I used to and my pace time for a 400 yds. is about 10 seconds slower than it was just a few years ago. I haven't changed anything in my training, but I must admit that all I do is aerobic work and anaerobic threshold work - no quality or sprints. Has this happened to anyone else? Any way to change things and get my faster self back? Thanks!