Uh... no they weren't.
If you're setting the benchmark as the signers of the US Constitution, I disagree with this assertion. For the sake of argument, let's also include the signing of the Declaration of Independence, which is IMO a much more idealistic--and traitorous thing from the perspective of the crown.
Of the 40 signers of the constitution about 8 were below the minimum age requirements of US President (35) [1] with an average age of about 44. Of the 66 signers of the Declaration of Independence, about 7 were below the minimum age requirements [2]. The rest were at or above 35, and there were many men in their 60s and 70s. These numbers are not exact as I'm going off quick calculations and a spreadsheet drafted in a couple minutes.
[1] http://hercules.gcsu.edu/~hedmonds/U.S.%20Constitution/Const...
[2] http://www.archives.gov/exhibits/charters/declaration_signer...
Edit: Missed one and bumped the signer count.