Benjamin Franklin was one of the oldest at age 70. Marquis de Lafayette was one of the youngest at age 18. It may be surprising but many of the Founding Fathers were in their 20s or 30s. We tend to think of them as being old guys because their official portraits were painted much later than 1776.
Take a guess at the ages (as of the signing of the Declaration of Independence) of these Founding Fathers: John Adams, Aaron Burr, Nathan Hale, Alexander Hamilton, John Hancock, Patrick Henry, Thomas Jefferson, John Paul Jones, James Madison, John Marshall, James Monroe, Thomas Paine, Paul Revere, John Stark, George Washington – then get the answers at the link.