Of course you can do aerobic activity any time. 

However, surveys have shown that those who get regular exercise and stick with it generally get their exercise in the morning. 

Those who are working full time jobs that require office hours usually get up early to exercise before work.  Those who have control over their schedules, like homemakers, the self-employed and retirees, usually exercise mid-morning.  

There are a couple of reasons for this.  One reason is that aerobic exercise gives you energy (if you are not overdoing it) and most people like to have that energy in the morning to use throughout the day.

The other reason is that the later in the day you exercise, the easier it is for your exercise time to be preempted by some other “urgent” situation and the more tired you are and thus, the less motivated to exercise.