Peak age for performance may vary between player or position, but it tends to be from the mid to late 20's.
Averages are often used to describe a squad or team, but these are almost always misleading.
A team can have an average age of say 26 which is made up almost entirely of peak age players aged 26 years or it could have the same average age, but comprise of near equal numbers of players aged 21 and 31.
These distributions can be expressed numerically, but the visualisation of a plot based on proportion of playing time given to each age group probably best illustrates the career cycle of the current crop of internationals.
The team who have access to the biggest proportion of peak aged players at the 2016 finals is Ukraine.