On February 28, 2028, this person will be 10,226 days old.
Here's the count. From February 29, 2000 to February 29, 2028 is 28 years. That span holds seven leap days (2004, 2008, 2012, 2016, 2020, 2024 and 2028), so it's 28 × 365 + 7 = 10,227 days. February 28, 2028 is one day before that anniversary, so subtract one: 10,226.
Two notes. If you count the birth day itself as day one, the number is 10,227. And for ages in years, the person turns 28 on February 29, 2028, so on February 28 they're still 27. Leap-day babies often celebrate on February 28 or March 1 in non-leap years, but 2028 is a leap year, so that question doesn't come up this time.
I did this by hand, not with a search, since it's simple calendar arithmetic. Anyone who wants to check it can use any date-difference calculator.
