I have completed the ages slightly out of order and now I find I can't get to the last age because the gates on both sides of this island on Direbo are locked. The order I did them in is:
Todelmer
Laki'ahn
Tahgira
and now I can't access Noloben. How can I link here to start this age?

It could mean two things.

1- You did find the linking/book to Noloben/Derebo in the Great Shaft but when you did get to the surface, you forgot to unlock the two doors.

2- You did not find the Noloben/Derebo linking book in the Great Shaft, so you never had a chance to unlock the doors.

In both instances, you will have to go back inside the Great Shaft, find that book, link to Derebo and unlock the doors.

Sorry for the bad news.