Hopefully this helps some:
(Explanation followed by Example)
*Explanation of the code you found:
=DateDiff("yyyy", [Birthdate], Now())+ Int( Format(now(), "mmdd") < Format( [Birthdate], "mmdd") )
=DateDiff - This tells Access you want to see the time between two dates that you're about to specify.
"yyyy" - Tells Access the interval of time you're looking for. In this case "yyyy" means you want difference in years. If you wanted difference in months use "m" or days "d" (only one letter for those...years is 4 y's). You DON'T put anything in it's place...if you want your answer as years you leave it as "yyyy".
[BirthDate] - This is your Field Name from your Table for your starting date. In your case as you described above, you would use [BDate] instead of [BirthDate].
Now() - This is pretty literal to Access. It refers to right this moment. You would leave this alone because if you wanted to find how old someone is then you would want to compare their birthday to today.
Int - Basically saying you're dealing with a number.
Format(now(), "mmdd") < Format( [Birthdate], "mmdd") ) - Basically this tells Access you want this expression to take today's date, the Now(), and find the difference with today's date and your Fields date, the [Birthdate] Field column. Again, leave the "mmdd" alone. It tells Access the date format to do the math for you. And yes I know it has double m's and d's when I just mentioned above to use only one of those letters for m or d.....it's a different area and here it's ok.
**Hint: When in Access hit the F1 key to open the help window. Type in the search box that pops up something like Now and it will give you the definition of it and tell you what it does.
***Example of using the code you found:
You are starting with:
=DateDiff("yyyy", [Birthdate], Now())+ Int( Format(now(), "mmdd") < Format( [Birthdate], "mmdd") )
Now, you said your Field Name in your Table was BDate. So lets update our formula:
=DateDiff("yyyy", [BDate], Now())+ Int( Format(now(), "mmdd") < Format( [BDate], "mmdd") )
That's it! Your formula is complete!! Access will do the rest for you!
Here's how:
Again, you said you already have a Table with a Field Name (a vertical column in your Table), labeled:
BDate
So we'll move on.
To get the diff in dates, and to use that data somewhere, you'll need to make a Query:
-In Access 2007 or 2010 with your database open, you can simply click the "Create" tab at the top of the Access program.
-click "Query Design"
-the "Show Table" pop-up box should have opened. Select the Table that contains your BDate Field and hit "Add" to add it to the Query Design window. Close the pop-up box.
-Double click something that identifies who you're calculating age for, such as a Field that contains a contacts name or whatever. Dbl clicking adds that field to the area at the bottom.
-Double click the "BDate" Field name to also add it to the bottom so you can see the birthdate when the fields are calculated for you.
-In the area at the bottom you should have several blank columns next to the information you just added to it. In the next available blank column, in the first row (Field), copy and paste your formula in: =DateDiff("yyyy", [BDate], Now())+ Int( Format(now(), "mmdd") < Format( [BDate], "mmdd") )
-Make sure the "Show" box is checked a few rows down so you can see your answer.
-Save your Query, and hit Run. You should see your ages next to your BDate.
-Use the results of the Query in other areas like Forms, etc.