Tom Holland is 23. At that age, there isn’t much “dramatic” aging as long as you take care of yourself. At 13, no matter what you do there will be dramatic aging between 13 and 15, for example. Tom is well past that stage of development.
Things haven’t been announced yet, but I think that it’s pretty clear that Sony is going to want a Spidey movie at least every two years, and Marvel is going to want to keep their arrangement and meet that timeline because it is good for the MCU.
I think it is very likely that Marvel will have to continue this story with not too much time in between Far From Home and “MCU Spidey 3”. Even if it is 2 years in the “real world”. Next year, what is likely in the MCU? Black Widow and Eternals, right? One is a prequel and the other is either off world and/or a prequel of sorts.
Shang-Chi isn’t likely to intersect with “modern day”
New York. Black Panther 2, Guardians 3, etc.. aren’t going to intersect with modern day New York. I’m not sure I see much else coming out within the next 2 years that will focus on “present day New York” other than Doctor Strange 2, and even that will likely be focused on its own thing. So I don’t see any reason why Spidey 3 can’t come out in 2021 and pick up pretty closely from the end of Far From Home.