I think the important thing is it's theoretical. You potential is at it's highest then but obviously if you train better in your 30s (or later) than you did in your 20s then you'll peak in your 30s.
But in terms of potential to build muscle (so strength, speed, looks) your potential to pack it on is highest through the first half of your 20s so your peak is somewhere around your late twenties/early thirties once you've got accumulative gains going on. From then most will (if judged by averages) loose muscle but fairly slowly due to declining growth hormones, as in a few kg a decade, assuming you continue to train. The decreasing hormones makes it easier to store fat and harder to build muscle so the work to keep where you were when you were 30 gets harder to keep up to the point that eventually almost everyone just can't so they decline
Depends what you mean by fitness - A quick glance at a few different sports shows you how much - gymnasts may well peak in their teens, footballers in their 20s and bodybuilders or weightlifters may well be older when they reach their peak.
Men hit their peak muscle mass in their 30s. After that, muscle mass declines. That decreases the body's ability to burn calories. Unfortunately, that inability to burn calories means you could be sporting more spare tires around your waist than the Michelin Man.
I've read multiple times that it's around 30 for men, perhaps late twenties. However, many have said that they've been the 'fittest they've ever been in their life' well into their late thirties, and even into their forties.
Depends what in, for tennis, for example, it's early twenties.
For most it's actually early twenties but when factor in mental factors, it can be a lot older than that as for sports "games" llike football where it's late twenties because positional play and technique come into play.