Issue with DATEDIFF

All,

PBI newbie here. Maybe I'm missing something, not real sure, but I have a measure that I'm using to calculate the average age of an opportunity from our CRM data. It works when coupled with opportunities that have both a createdon date and an actualclosedate; these are both columns in my table. I created another column with the Today() function as the value. What I'm trying to accomplish is show the average age based on the dates(createdon vs today's date, or createdon vs actualclose date) of each opportunity. Thoughts?

Current working formula: 

Avg Age = AVERAGEX (opportunity, DATEDIFF ( [createdon], [actualclosedate], DAY)

Tried:

Opp Avg Age = AVERAGEX (opportunity, DATEDIFF ([created on], [today], DAY) >>> The result I get is ever single column added up and averaged (so instead of individual number of days per opportunity, it's just entire column added and averaged)