I replied to this but my reply seems to have been deleted. =:(

Melvin Doo. You sound like a relative. The Hanna-Barbera Wiki lists all of them at 25 although the classic comeback of a captured antagonist of "meddling kids" led me to believe they were teenagers.

Velma - October 31 (year unknown)
The others, I don't know. I haven't been feeling well. Doctor changed my antidepressants and I'm getting a serious fallout until my new ones begin. Otherwise I'd have the strength to research if further. Good luck though.

The gang's birthdays have never been mentioned. Velma's COUSIN MARCY has a birthday on Halloween, but not Velma herself. And that's only in the What's New series. So, nothing canon for the gang yet. = )

Scooby Doo birthday is September 8th. That was from the 1984 series "The New Scooby Doo Mysteries ". Episode 1 titled "Happy Birthday Scooby Doo".