This is a limitation of Google Calendar. Events older than 12 month are usually only available online in the web browser at Google Calendar

Since aCalendar is not involved in synchronization, we have no influence on this. 

Ideally contact Google about it, the more people contact Google, the more likely it is, that this limitation will be lifted.

Workaround 1:

To manually synchronize old events to Android, you can use the following workaround (needs to be repeated every 12 months):

  1. Go to Google Calendar on a PC in a web browser
  2. export your calendars (Export your Google calendar)
  3. re-import the calendars (needs to be unzipped before) (Import events to Google Calendar)

The re-import will update the modified timestamp which is used by Google to determine if the event should be synced to mobile devices or not.

Unfortunately, we are not aware of such a solution for future events that are more than 12 months in the future.

Workaround 2:

You can sync to Google Calendar without this limitation with CalDAV Sync, but it loses all extra features of Google Calendar (appointment colors, email notifications, push sync).