[Someone]

But “age is 42” is a statement/Boolean, not a mutation. “Let age be 42” is more natural, but doesn’t sound well on reassignments (“let age now be 42”?)

[samatman]

I would say this as "set age to 42".

[postalrat]

"age is 42" may be a statement while "is age 42" may produce a boolean.

And I don't see why a statement can't describe a mutation.

[hcs]

I read assignment like "age gets 42".