Kasraouia's formula didn't work for me.
After much digging, I found a solution via "Power Platform Boost Podcast" on YouTube. (https://www.youtube.com/watch?v=lTHQR4xiHio)
NOTE: Not endorsement implied.
His formula returns values even when a 'Date of Birth" cell is blank, so this formula (courtesy of ChatGPT) returns "No Birth Info" when a cell is blank.
If you'd rather it return a number, like a zero, just replace the "No Birth Info" with a 0.
However, do so in a text editor before you paste it in to the formula field. I'm not sure why, however once it's in the formula field, any typing to edit breaks everything.
If(
IsBlank('Date of Birth'),
"No Birth Info",
If(
(Month(UTCToday()) > Month('Date of Birth')) ||
(Month(UTCToday()) = Month('Date of Birth') && Day(UTCToday()) >= Day('Date of Birth')),
Year(UTCToday()) - Year('Date of Birth'),
Year(UTCToday()) - Year('Date of Birth') - 1
)
)
