Alexandre Pato, Adriano, Saviola, Anderson, Victor Valdes, Rooney, Denilson, Kaka, Torres... plenty of players have peaked in their early twenties, especially those who suffer injuries and rely heavily on their physical attributes like speed and agility.
There is no guarantee players will keep getting better in their mid to late twenties and actually often suffer a decline in performance if injuries take their toll.
Rashford's been rather poor aside from that 2-3 month great period last year and he honestly doesn't look fit or confident anymore. Really worried about his back, it took Van Persie 3-4 YEARS to fully rehabilitate after his injury and he had to dedicate his entire life to staying fit, doing yoga and getting extra, personal trainers to stay fit and only managed to do so when he was 27-28.