Pretty simple question.
Uh . . . do you mean younger than I am now, or younger than I was when he was President?
Younger then you were when you were President
Younger than you at the time when he took the oath of office.
According to wiki, the median age in the US is 36.7 years, so that’s probably going to lead to a lot of “all of them have been older” answers. A fair number of dopers aren’t old enough to be president themselves, and no president has ever been anywhere near as young as 35, so…
The current one, President Obama is younger than I am. All other presidents were older than I was at the times of their inaugurations.
I, too, interpret the question as “of the presidents in your lifetime, which one was the first to be born after you were”.
Which gets me Barack Obama, by barely a two-week margin! Like others mention, I expect a huge majority to answer that there has been no such case yet, and the #2 spot be carried by Mr. Obama. But I expect quite a number of currently 65+ Dopers will get Mr. Clinton.
None yet, though Obama is slightly more than 10 years older than me, so it will happen in the next few elections.
Hey! Maybe I’ll be the first President who’s exactly the same age as me!!!
Vote jk1245 in 2016 (or maybe 2020)!
That would be Mr. Obama, who is about 18 months younger than I am.
None, yet. But it’s coming soon as I moved into my mid-40s.
When it happened with baseball players it was a painful event.
All older so far, but Obama came very close. He’s less than three months older than me.
None yet, but it’s getting closer now. The present president is about 3 years older than I am.
To make it simple, keep going down the list until you find the first one that applies:
If you were born after August 4, 1961, no President has been older than you.
If you were born after August 19, 1946, Barack Obama was the first President born after you.
If you were born after October 1, 1924, Bill Clinton was the first President born after you.
If you were born after May 29, 1917, Jimmy Carter was the first President born after you.
If you were born before May 29, 1917, John F Kennedy was probably the first President born after you unless you are extremely old.
I’m 32 at the moment, so nobody yet. 
When is your birthday?
No one, yet. I’m 27, so it’s going to be awhile before that happens.
Discrimination! I demand that Teddy Roosevelt be added to this list!
Actually, I’m just a little older than Clinton. I doubt if there will be any older than Carter here.
Obama’s still got me by 2 years. So no one yet.
Nobody so far, but Obama is close. He’s less than a year older than me.
Actually, I think it would be Franklin Roosevelt who’d be next on the list. But I figured there weren’t going to be too many people here born before January 30, 1882.