Derby manager Nigel Clough admitted inexperience at the back cost his team dearly as the Rams lost 2-1 at Brighton.
Jeff Hendrick's late goal was too late as County were beaten for the sixth time in their last eight away league games.
He said: "Mark O'Brien was caught out of position for the first goal and the manner of the second goal was disappointing.
"There was a foul on Richard Keogh prior to Brighton scoring but the referee missed it. We had no decisions in the first 30 minutes of the game.
"We had two inexperienced 20-year-olds at the back and didn't have the nouse or experience that Brighton had in their side.
"But I never felt that we were out of it, even at 2-0 down. We could have been 4-0 or 5-0 down. But it wasn't and we were very spirited."
Clough was disappointed that Derby's final ball led to them not creating too many chances.
He confessed: "Our youthfulness at times will be our undoing. I didn't see too many teenagers in the Brighton side and their players know their jobs and know the game.
