He told chelseafc.com: "I have been fortunate enough to jump around the different teams this season. It was tough to begin with as I needed to learn how to adapt to the different coaching styles, but I soon settled in.
"I believe there is a slight jump from the U18s to the U21s. The main difference is the speed, everything is a lot quicker with more power and aggression. As a result, I have improved my ability to react quickly and always remain alert."
Merrick also sat on the bench for the FA Cup tie with Aston Villa this season.
He added: "I have been fortunate enough to be a part of the men's matchday squad, being involved in training and a fixture.
"Training with the men's team was an unbelievable experience. It was great to have a taste of what the next level is like, and it has only driven me even more to get there permanently.
"You don't get those opportunities often so I made the most out of it. I looked closely at the current Chelsea keepers and took on all the advice and comments that the men's coaching staff gave me."