I planted a couple of Maples just like them. Mine are about 14' apart.
Why? Only because I do what I want.
They are planted to block the sun from shining on my vehicle from about 2pm and later.
I want the shade where I need it in 2 years.
We will probably build a carport in 15 years. So if the trees need to come down in 20 to 40 years, no big deal.
They are still far apart that they can be trained decade, after decade, after decade.
Sometimes we plant for trees to keep. Other times we use them as "green tools" following the concept of "trees, our renewable resource."
Trees need to be evaluated from at least 2 aspects:
1. Trees to keep.
2. Trees that serve a purpose for a predetermined time:
a. screening
b. firewood
c. shade
d. material for crafts and woodworking
e. lumber
f. grown to die - nesting for certain animals
g. for research
Mario Vaden
Designer / Arborist
M.D. Vaden Trees & Landscapes
Beaverton, Oregon