Custom ranges can be created a number of ways such as using a SQL query, Excel file, R or Python Script.  I am going to show you how you can do it using a Calculated Column.  Calculated Columns use the PQL language.  Refer to the help file for more information on the PQL syntax (LINK).

Here is the PQL syntax to do what you are asking:

case(
criterias(
 ([Age]<18)
,([Age]<26)
,([Age]<40)
,([Age]<55)
,([Age]>=55)
)
, dataSet(
  "0-17"
, "18-25"
, "26-39"
, "40-54"
, "55+")
,"na")

Here is a screenshot showing the code in action:

Here is an example using Python.  

Steps

• Add a Python Node
• Paste the following code in a quick script

# check age value

ageGroup=[]
for i in range(0,len(ageColumn)):
    if (ageColumn[i] < 18):
        ageGroup.append("0 - 17")
    elif (ageColumn[i] < 26):
        ageGroup.append("18-25")
    elif (ageColumn[i] < 40):
        ageGroup.append("26-39")
    elif (ageColumn[i] < 55):
        ageGroup.append("40-54")
    elif (ageColumn[i] >= 55):
        ageGroup.append("55+")
    else:
        ageGroup.append("NA")

DataFrame1= pa.DataFrame({'AgeGroup':ageGroup}
)

• Configure Inputs and Outputs