Yes, and it's even happened before. John Quincy Adams was a Congressman from Massachusetts after his term as President.
head of state
head of state
John Quincy Adams, 1831-47
Ambassador is a representative of the President to other nations.
The President pro tempore
Representative Government
He served as a representative from 2012 to 2014.
A representative must be twentyfive years old and an USA citizen and he/she must serve for two years.
Representative Government
They serve for two year terms.
I believe you are referring to the minimum age requirement to be a representative, senator or president. You must be at least 25 to be a representative, 30 to be a senator, or 35 to become president.
2 years