Let's say, you have an event (a moment in time) in the future. Let's also say, you want to associate something with that moment. Since a moment does not last (or we can presume that it does not), it moves immediately from the future to the past, in a blink of an eye, so to speak.
In other words, it's in the future now (she will turn 22 at some moment in the future), but right after that moment, her turning 22 will become past, right? And soon after that moment something else will happen (or will become obvious), and that you want to describe, like her accommodation.
So, to re-iterate, with respect to the accommodation, the moment will be in the past. But now it's in the future. So, when we speak of the accommodation and the moment of her turning 22 together, the turning 22 is "past-in-future".
To express "past-in-future" we use present tense:
When she turns 22, she will get her accommodation.
Here, "turns" is present tense, 3rd person singular that denotes "past-in-future" with respect to the main clause of the sentence.
To turn this into interrogative, you put the main clause first, then the subordinate clause:
Will she get her accommodation when she turns 22?
The subtle differences in meanings can be recognized when instead of the auxiliary verb "will
" the verb "shall
" is used. Compare:
Shall she get her accommodation when she turns 22?
means to ask about the legal status of getting the accommodation. The rules or university code or by-laws might state (literally) that "no student shall get their accommodation until the age of 22", for instance, which mandates certain order, certain behavior, certain relation between getting the accommodation and the student's age. It can also be expressed in the negative, "students shall not get accommodation until the age of 22". If for some reason the rules do not state the age, a question aimed to clarify the rule can be asked with the use of the same auxiliary verb:
When shall she get her accommodation?
At what age shall she be eligible for accommodation?
In normal circumstances we rarely use "shall" to refer to the future, unless we are certain or we are asking permission of (or making a suggestion to) somebody in charge:
Shall I boil the water now, master? -- Yes, Johnson, please do!
Are you ready, darling? Shall we try for the next train?