What is the cutoff age for a dependent

It depends.  He would not be considered a qualifying child as the cut off age is 23 (UNDER 24)

If his total income was less than $5,050 you can still claim him as a qualifying relative.  If his income was greater than that, then you would not be able to claim him at all. 

Where is the option on your return for qualifying relative?

When you walk through and answer the questions on your son, it will give you determine if he qualifies for the Other Dependent Credit.  It is not a checkbox on the forms. 

Basically, you do not do anything different than if he was a qualifying child, you just answer the TurboTax questions in regards to him and then you can claim the credit and head of household if he qualifies. 

Qualifying Relative