Ygtjki

In conversation, I said incredulously and jokingly:

Comment peux-tu être trois ans plus jeune que moi et pourtant si mature ?

This phrasing sprang to mind in the flow of conversation, but now I think back on it, I wonder if it might have been better to place the part "si mature" first, given that this is the surprising bit that should be directly and immediately emphasised by "Comment peux-tu":

Comment peux-tu être si mature alors que tu es trois ans plus jeune que moi ?

or: Comment se fait-il que tu sois si mature alors que tu es trois ans plus jeune que moi ?

Hard to give an objective answer to this question. All sentences sound good and equivalent to my ears (aside from the pouvoir/se faire distinctions).

I would probably tend to put the fact I learned last (what you call the surprising bit) first in the sentence, unless I want to make it sound like a deeper thought.

The last two constructions are better because they are more natural and that is so on the count that it is more logical to wonder about the unusual maturity than the age, which after all is known ; to doubt the age of someone is proper when there is an incertitude or when the age appears to be incredible. In the present case and such other cases, doing that is awkward; at least, it seems so to me. The surprising bit, as you say, should be the focus of your enonciation (at least in the present sentence).

By the way, « mature » has been an English word until recently; it is not yet in the TLFi, not in the Ac., not in the Littré but it is found in the on line Larousse; a periphrase can be used to replace it, though.

• Comment peux-tu avoir tant de maturité alors que tu es trois ans plus jeune que moi ?

There is another problem; maturity is not considered as a quality that depends on age according to a proportionality factor that should be the same for everyone; moreover in the French culture, maturity is not something that is developed in a human being all his/her life so as to be for ever growing until death; wisdom may be considered such a caracteristic or certain skills; in the French culture it is considered that full maturity is reached once and for all at a point between youth and old age and that it remains constant for the rest of adulthood, a period from which is excluded very old age. On top of that, a mere 3 year difference is not a factor worth considering; here is the definition of the « age of maturity » or, elliptically of « maturité » ;

(TLFi) Âge de maturité; p. ell., la maturité. Période de la vie comprise entre la jeunesse et la vieillesse pendant laquelle les facultés humaines ont atteint leur ultime développement.

I believe that the idea is not expressed in the proper terms. Maybe it is preferable to consider a quality such as wisdom and not to make it so dependent on age.