=IIf(IsNull([DOB])=False,Date()-[DOB],Null)/365.25
This means that IF there is a DOB value, return Date()-[DOB] otherwise return Null / 365
You cannot do math on Null, nor can you compare it to anything (e.g. = Null)
I think you want
=IIf(IsNull([DOB]),put value if nulll here, expression if not null here)
EDIT - forgot the main question
I doubt you've run into a record where the field is null otherwise you should have generated an error.
You can use Format function to trim decimals, or you can use a conversion function if you don't want any decimals at all (e.g. CLng)
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.