Maximum frequency is dependent on VCC. As shown in Figure. Maximum Frequency vs. VCC, the Maximum Frequency vs. VCC curve is linear between 1.8V < VCC < 2.7V and 2.7V < VCC < 4.5V.
Figure 1. Maximum
Frequency vs. VCC for ATmega48V/88V/168V
Figure 2. Maximum Frequency vs. VCC for ATmega48/88/168