Change your script part to below.
Load *,num(interval(task_closed_at-[Link Date]),'#,##0') as age;
//if(isnull(task_opened_at), task_closed_at,task_opened_at) as task_opened_at,
if(Isnull([Link Date]),task_closed_at,[Link Date]) as [Link Date],
This seems to be a cross-post of exactly the same issue on the same application made by two different members... Really strange. Please don't cross post - it waste a lot of time for others trying to help and it is very disrespectful.