Pfft, their *age* goes up the same as anyone else's, it's their number of Birthdays that is less .
Also, not every year divisible by 4 is a leap year. Last I heard the rules are:
1) If the year is divisible by 400 -> leap year
2) If the year is divisible by 100 -> NOT a leap year (but see rule 1 first)
3) If the year is divisible by 4 -> leap year (but see rule 2 first)
4) Any other year isn't a leap year.
Hence 2000 WAS a leap year (rule 1), but 1900 wasn't (rule 2), and this year is (rule 3), but next of course won't be (rule 4).
All this mess because the time it takes the Earth to do one orbit of Sol isn't exactly 365.25 days. If it was dead on 365 we wouldn't need leap years at all. In general the Earth's rate of rotation slows down so eventually it will be 'exactly' 365 days and anyone born on 29th Feb will probably never celebrate a birthday again .