I wouldn’t be surprised if he was quite broken by that age; most truly rapid players are done by that age or have emphatically modified their game; if not knees, then generally hamstrings destroy them and on the odd occasion, it’ll be the back that can no longer function.
Haaland already makes uncoachable runs and instinctively knows what to do and when, so I’d expect he can modify that aspect to not rely as heavily on pace, but being clean through for shots off what would have once been from those same runs will diminish, as we’ve seen with so many others.
Shearer was never as fast as Haaland, but he was once reliant on his pace to work angles for himself and played a very clean, breakaway game - he modified his game so much when he lost his pace, people who didn’t see his younger iteration swear he was always a hurly burly bruiser. Maybe Haaland will learn to use his body in the more conventional ways once/if his pace and athletic prowess diminishes. Relative to his size, he’s dire in basic Zlatan-sequel tussles and scrambles at the moment, but if forced to do the nitty gritty like so many others had to beyond a certain degradation, we’ll see what comes of that.