"Is there any syntaxe in matlab that allows me to do that ?"
You could use the fieldnames of a structure:
data.y2021.Bob.age = 43;
data.y2021.Camille.age = 55;
but a much better would be to use a table or non-scalar structure (and not force meta-data into fieldnames), e.g.:
S(1).year = 2021;
S(1).name = 'Bob';
S(1).age = 43;
S(2).year = 2021;
S(2).name = 'Camille';
S(2).age = 55;
