Varies person to person. But generally.
Speed, flexibility and agility: early 20s
Raw power: around 30 (some it even seems to increase all the way until nearly 40)
Strength: 35 (this is a weird one because so much is due to how well your joints and body holds up, your muscle begins to decline naturally without exercise at 25, but if you work out often some are at their strongest at 35 or more) Then there is old man strength, which is another weird one. That is probably more due muscle memory though.
Honestly, I'm 34 and the only two things that have notably declined for me is my recovery time (which is only slightly noticeably worse than my 20s) and I gain fat waaay more easily than when I was a human garbage disposal with 10% body fat in my 20s.