DateDiff takes three arguments: the type of difference it is to return, and the two dates it is to compare. You have these, but you are not separating the arguments with commas. There is also no need to subtract one date from the other - that defeats the purpose of using Datediff.
YearDiff: DateDiff("yyyy", [Visit Date], [DOB])
If the date difference in years returned is negative just swap the visit date and DOB arguments over.
-Stewart