For example. If I highlight cell C1 and go to Data|Validation and change the Allow: value to "List" by clicking on the down arrow and typing "<25,26-45, 46-64, Over 64" into the box labeled Source (without quote marks). Then clicked ok. This will make it so when I click on C1, a drop down arrow appears allowing me to choose one of those options.
Then in another cell (in my test file I used C3) I used the following formula,
=IF(ISBLANK(C1),"",IF(C1="<25",1,IF(C1="26-45",2,IF(C1="46-64",3,4))))
What this does is first, checks to see if it's blank, if it is, it leaves C3 blank. Then it checks to see if the value in C1 is <25, if it is, it puts a 1, if it's not, it checks for the next option, and the next, etc until you have your 4 options covered.
Is that what you're looking to do?