If you multiple it by 12, you will get months:
((decimal-date-time(today())-decimal-date-time(${birthday})) div 365.25)*12
You can use this reference guide to tweak the result with either int() or round() depending on what value you want.
Formulas—Survey123 for ArcGIS | Documentation
You could get really fancy with if() and calculate years if the birthday is further away than 1 year or months if the birthday is less than 1 year in the one field, though you may want it to be a text field so you could append "months" or "years" on the end so it's not confusing.
if(condition, a, b) | If true, returns a; otherwise, returns b. |