:Ribbon Salminen, 
 ::Thanks for picking up on the errors. 
 ::Yes, {{age in days nts|month1=02|day1=08|year1=2015}}+ does work perfectly well but do you really think it's easy to understand?  {{age in days|February 8, 2015|sortable=on}}+ seems so very much clearer to me; I mean, don't you agree?  Okay, the  bit is a little obscure but it's used by  (which is a commonly used template) and it's arguably less obscure than tacking nts on anyway. 
 ::One can make a typo when entering numbers as well as when entering letters.  Actually, it's nice that you pointed this out, though, since the this is another benefit of doing thing the way I'm proposing.  A spelling mistake will give a visible error message whereas the wrong number will just give a wrong output. 
 ::Yes, people might forget to put  but they might forget to use  and use  instead.  You know, what I find?  I find it easier to cope with fewer templates doing essentially the same thing but with a few minor (or sometimes not so minor) differences handled using parameters than to cope with a plethora of them going off in all different directions. 
 ::I'm aiming to make it easier for users by simplifying and standardising the use of these templates.  This means fewer templates and more natural input.  
 :Thanks again for your thoughts.