Don't know about slowing down, I am fifty and I am used to very intense workouts and having more than three of those per week is hard. As you get older I seem to need more time to recover and there are a lot more injuries. Like my left elbow has some tendon that feels funny, my knees are no longer able to take the volume of squats I used to do and my lower back is sore regularly.

But when I workout I still feel like I am twenty... I still can do 20+ pull-ups with good form and without stopping, I can easily do 100 pushups and I am still fast at sprints. I look like I did in my 20s and 30s muscle wise, but it is not the same... the body is just more fragile, I am no longer indestructible.