Today, 5 April, seems to be Satoshi Nakamoto's (symbolic) birthday. Congrats.
Ning/P2P Foundation requires a birthdate for signups, and displays for every member an age calculated from that birthdate. This is the basis for ages given for Satoshi. However, the age changes each year; for example:
Since the displayed age yesterday (4 April 2014) was 38, and today (5 April 2014) it is 39, I infer that his birthday is 5 April and his birthdate is 5 April 1975 (2014 - 39).
There is, as far as I can tell, nothing special about 5 April; it's not a round number, it's not a symbolic date, it's not your usual fake birthday like 1 January or April Fools, it's not the day Satoshi signed up for P2P ("Satoshi Nakamoto is now a member of P2P Foundation Feb 11, 2009"), it's not related to when Bitcoin was released (January) or when the domain was registered (August) etc etc. So it seems like a good guess at a birthday.
EDIT: edlund points out I missed an entry in the Wikipedia list which might be very important to libertarians:
On April 5th 1933 U.S. President Franklin D. Roosevelt signs two executive orders: 6101 to establish the Civilian Conservation Corps, and 6102 "forbidding the Hoarding of Gold Coin, Gold Bullion, and Gold Certificates" by U.S. citizens.
This raises another question: if the choice of birthday was symbolic, then is there additional symbolism in the choice of birthyear and/or claimed age when he registered? Is there anything special about 1975 or '34' in a libertarian context? edlund points out there is for 1975, and in fact, it's directly connected to the April 5 fact:
Another important thing about the year 1975 - it was the year in which gold ownership was legalized for the mere mortals in the US
I find this pretty convincing. Well played, Satoshi, well played indeed - even now, >5 years after you registered that profile, we're still finding easter eggs you left for us.