This is a limitation of Google's calendar sync on Android. Events older than 12 months (and events more than 12 months into the future) are usually only available online in the web browser at Google Calendar.
Google Calendar seems to be experimenting with extended but proprietary sync - unfortunately, the data is locked in and not shared with other calendar apps like aCalendar.
Experimental Solution (requires aCalendar+ or Business Features)
You can set the future sync time window to 1, 3 or 5 years and you can request re-sync for the events of the past 3 calendar years. This is offered in the calendar list (MENU - Manage Calendars) in the 3-dots menu for each Google Calendar (though the future sync time window is actually set per account).
Manual workaround (past events only):
To manually synchronize old events to Android, you can use the following workaround (needs to be repeated every 12 months):
- Go to Google Calendar on a PC in a web browser
- export your calendars (Export your Google calendar)
- 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.