We have a custodial agreement to alternate years for our child. He turned 18 in the first part of 2016 and I wonder if the agreement is valid through 2016?

We have an agreement through the court that we alternate years to claim our son.  2016 is my year but my son turned 18 in early 2016.  Is that agreement still valid?  My ex husband  became the custodial parent for the 2015-16 year and claimed him in both, however the every other year agreement was never altered.