What is the performance impact of updating fixlength non indexed field vs var length field

Does an update in place occur if an update only affects a fixlenth field?

create table ages ( Id integer, age integer);

insert into ages values ( 0, 20);

insert into ages values ( 1,40);

---  does a the row get removed from the data page and then re added 

-- or does the value of just  age change

update ages set age = 42 where Id = 1;  

????  and is there any doc that explains what happens

Howard Davis