You would use the DateDiff function (https://www.techonthenet.com/access/functions/date/datediff.php)--3 different times. Once calculating year, once calculating month and once calculating day. Then for the month and day you would have to do math (/12, 1/365, etc) to get the exact number you want.
Hi. Since you already have a working solution from Pat, I guess we don't have to pursue this anymore. However, saying "it didn't work" wouldn't be enough information for us to help you troubleshoot the problem, if you had wanted it fixed. So, for future reference, let us know exactly how the suggestion didn't work and provide any error message you may have received. Good luck with your project.
If you choose to use the code in my sample (or whatever code you want to use), you can put it into your own function and have the function return the text string you want. Call the function from the query.