I have a DOB(date) field and a Decimal Field(Custom) that are already created and fullfilled. 

What I need is a new and automatic field that returns me the following: 

If the actual age is <= 50, returns to me the result of my (Decimal custom field) / 0,0048) or

If the actual age is >= 51 and <= 59, returns to me the result of my (Decimal custom field) / 0,0018) or 

If the actual age is >= 60, returns to me the result of my (Decimal custom field) / 0,0008).