The 30-year-old retired from international duty with over 100 caps after helping Germany win the World Cup in Brazil this summer.
He told the club's official website: "It's tough to establish yourself as a young player at Arsenal as a lot of the positions have two players in, so it's hard for the youngsters to impress in regular games.
"But we have to rely on our reserve players and it's good to get a good relationship with them and try to give them some help. The most vital part is good communication on the pitch – they have to talk to me if there's someone in my back.
"I have to encourage them to speak to me even though I'm 30 and have a lot of international caps. They have to be brave and be themselves."
Mertesacker joined Arsenal from Werder Bremen in 2011.