I don't think age is particularly the right gauge for a goalkeeper's experience, more relevant is that he's only played 7 games in the football league. Under 21s, Nuneaton loans, great but he needs to play regular league football with us to reach his potential, whatever level that is at.
Players in any position usually find their first season of league football tough (look at Stokes), unfortunately when you're in goal a mistake more times than not is very costly.
Keepers tend to mature later than outfield players. Usually because they are waiting to fill dead mans shoes. Most outfield players can play in several positions so get game time more quickly. Also goalkeepers can usually prolong their careers for several years compared to outfield players.