I was always told to use fresh eggs when making meringues but was never told why.
Less than fresh eggs can be used, but as eggs get older changes begin to happen to its internal chemistry. For one eggs become less acidic as they age.
Meringues benefit from acidity:
Acid delays coagulation, which means that there is more time for air to get trapped in amongst the proteins, resulting in a lighter meringue.
Refer to this question which tackles the value of acidity in a meringue.
Other changes happen to the egg as well which may have an impact. Water is transferred from the egg white to the egg yolk, the egg white loses some carbon dioxide, and so on. All of this may have an impact, even if minor, on meringues. Using fresh eggs allows you to keep these variables in check.