Arsenal have a squad with young players and of course, I think that factor also has an impact on their mentality as well from time to time. Because after all, building a team to be strong in all competitions doesn't seem to only be about having young players. Because if the coach is not able to form the mentality of the players to become a winning mentality, then at least they will experience a decrease in performance from time to time. Thus, every player must have a winning mentality and excellent conditions, of course to be able to have consistent performance throughout the season.
Arsenal have big ambitions about whatever competition they enter. Arteta has built his squad well and improved his side to near perfection last season, but I agree they haven't got the winning mentality that one would expect. Even if Arsenal proved their quality last season, I believe they will never be the same next season. Opportunities can increase at any time for Arsenal and they can also decrease, it depends on how Arsenal form next season.
The Champions League is a different competition compared to the Premier League and other domestic competitions, so even though Arsenal have young quality players like now, that doesn't guarantee they can win the title without good experience in previous seasons.