Morales could appear fairly young in the MCU, having been dust by Thanos and not age for five years or surviving to grow to the same age as Peter. According to Donald Glover’s Aaron Davis in Spider-Man: Homecoming, his nephew seemed fairly young at that point. Additionally, there’s the question of this being a supporting role or a small cameo to set him up for future appearances in other MCU movies or Sony spin-offs.
According to Jeremy Conrad of MCU Cosmic, Sony and Marvel have already cast the role of Miles Morales. Conrad is not aware of who the actor could be or if this Morales casting would even be for Spider-Man 3, just that they have been cast and will appear in the MCU.
As Conrad points out in his report, this would be perfect timing for the character to appear in live-action. Not only was Miles Morales the star of an Oscar-winning animated movie with 2018’s Spider-Man: Into the Spider-Verse, but he will soon be front and center in his own video game from the same developer as Marvel’s Spider-Man. If the role has indeed already been cast, then it would only be logical for him to appear in Spider-Man 3, but in what form that would be is entirely speculation.