There is no SAS FORMAT named MMDDYYYY so it is not clear what type of variable you have the survey date.
Run PROC CONTENTS on your dataset and check the TYPE of the variable and what FORMAT, if any, it has attached to it.
If it is numeric with a format like MMDDYYN8. or DATE9. or any other format that works with date values then you already have a date value.
If it is numeric but has some other format attached, but prints as digits strings that a human would recognize as a date in MMDDYYYY style then you will have to first use PUT() function to convert it to a string and then use INPUT() function to convert it back into a number, only one that is an actual date value.
If it is character you will need to use the INPUT() function to convert it into a date value.
You can use MDY() with an arbitrary day of the month, like the 1st or the 15th, to convert your month and year of birth into a date value.
Once you have two date values then the yrdif calculations will be possible.
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